/**
 * MixVisualizer — the scrolling Mix waveform background (Phase 10 + Lava Reframe).
 *
 * What this renders: a *windowed* slice of a mix's loudness profile, scrolling
 * bottom-to-top, coupled to playback position. New audio enters at the bottom,
 * already-played audio exits off the top, and the "now" playhead sits at a fixed
 * line (vertical centre by default). This is a read-only, ambient lava-lamp
 * background — there is no seek, no click handling, no write-back to playback.
 *
 * Rendering tech: WebGL2, fragment-shader, plus (as of the Lava Reframe Wave R2) a
 * CPU-side per-frame PHYSICS step that drives the wax lava. The scroll/zoom geometry
 * and the loudness-datum-as-texture sampling carry forward from Phase 10 Waves 1–2;
 * the playhead wall-clock interpolation + jitter correction carry forward as-is.
 *
 * THE LAVA (Wave R2 — this revision):
 *   The rejected analytic-metaball "lava" (scripted blobs that read as giant
 *   disconnected circles) is replaced by a real Lagrangian wax-blob simulation:
 *     • 16–32 blobs carry position / velocity / temperature / radius and are
 *       integrated each frame with real dt (gravity, temperature-buoyancy, viscous
 *       damping, soft floor contact) — see stepPhysics().
 *     • 2D ELASTIC COLLISION: blob↔waveform (the ribbon is a read-only boundary the
 *       wax is pushed out of along its surface normal) and blob↔blob, both with a
 *       soft↔hard strength dial — see stepPhysics()'s collision passes.
 *     • The blobs upload as a uBlobs[] uniform array; the fragment shader unions them
 *       with smin metaballs + the waveform SDF into one liquid surface (liquidSdf).
 *   Colour is a deliberately SIMPLE theme fill for R2 — the OKLab three-colour
 *   gradient is Wave R3. No glass, no screen-space noise (removed in R1).
 *
 * The Blazor component owns the canvas element and the inputs (datum, playback,
 * zoom, theme, the control dials); this module owns the requestAnimationFrame loop,
 * the physics step, and all the GL math. The component drives it through the handle
 * returned by `create`. The handle SHAPE is unchanged from Phase 10 — the three
 * effect setters are temporarily re-routed to the lava params for this wave (see
 * their definitions); Wave R4 gives them proper names + a six-knob UI.
 */

// ── Tuning anchors (see spec §B). These are the load-bearing constants. ──────────

/**
 * Hard anchor: at maximum zoom the window shows exactly one quarter note at
 * 180 BPM = 60 / 180 s = 0.333 s of audio, top to bottom. This is a fixed
 * requirement, not a tunable.
 */
export const MIN_VISIBLE_SECONDS = 60 / 180; // 0.3333… s — quarter note @ 180 BPM

/** Slow end of the zoom range — how much of the mix is visible at minimum zoom. Tunable. */
export const MAX_VISIBLE_SECONDS = 30;

/** Default opening window when a mix is first opened. Tunable. */
export const DEFAULT_VISIBLE_SECONDS = 10;

// ── Control tuning anchors. These mirror the C#-side defaults in ──────────────────
// MixVisualizerControlState.cs — keep the two in sync, exactly as the
// DEFAULT_VISIBLE_SECONDS / DefaultVisibleSeconds pair is kept in sync. All three are
// normalized [0,1].
//
// R2 TEMPORARY RE-WIRING (Wave R4 replaces this with the proper six-knob set):
// the three existing control knobs are re-purposed to drive the new lava physics so
// Daniel can feel the system in-browser this wave. The knob NAMES on screen still say
// the old thing; the SETTERS below (setBubblyness/setDetach/setColorShiftSpeed) route
// them to the new physics params. Mapping:
//   • "Detach" knob (Air icon)        → lava HEAT
//   • "Bubblyness" knob (BubbleChart)  → lava GRAVITY
//   • "Color-shift" knob (Palette)     → COLLISION STRENGTH
// Blob DENSITY has no live knob this wave; it sits at DEFAULT_BLOB_DENSITY (R4 adds it).
// The defaults below are chosen so the lava looks ALIVE on open (heat non-zero, mid
// gravity, mid collision) — Daniel then tunes on screen.

/** Default GRAVITY dial (was bulge). Mirrors C# DefaultBubblyness. Mid = a settled-but-mobile lamp. */
export const DEFAULT_BUBBLYNESS = 0.5;

/** Default HEAT dial (was detach). Mirrors C# DefaultDetach. Non-zero so the lamp is alive on open. */
export const DEFAULT_DETACH = 0.45;

/** Default COLLISION-STRENGTH dial (was color-shift). Mirrors C# DefaultColorShiftSpeed. Mid soft↔hard. */
export const DEFAULT_COLOR_SHIFT_SPEED = 0.5;

/** Default blob density (no live knob this wave; R4 exposes it). 0 = few large lazy blobs, 1 = many small. */
export const DEFAULT_BLOB_DENSITY = 0.4;

/**
 * Where the "now" line sits within the window, as a fraction from the top.
 * 0.5 = vertical centre (default): a short lead-in below, a short trail-out above.
 * Tunable. NOTE: kept in sync with the GLSL constant NOW_ANCHOR_FROM_TOP below.
 */
const NOW_ANCHOR_FROM_TOP = 0.5;

/**
 * Half-width of the ribbon at full loudness, as a fraction of half the canvas
 * width (the predecessor used 0.92). Mirrors the Canvas `maxHalfWidth` factor.
 */
const RIBBON_HALF_WIDTH_FRAC = 0.92;

/**
 * Cap device-pixel-ratio at 2. Beyond that the extra fragments cost frame time for
 * no visible gain on a soft glassy backdrop — this is the graceful-degrade lever
 * (spec §5.1): drop internal resolution before dropping frames.
 */
const MAX_DPR = 2;

// ════════════════════════════════════════════════════════════════════════════════════
//  R2 — the wax-blob lava physics (CPU step + uniform upload). The lava is now a real
//  Lagrangian particle system integrated each frame on the JS side and rendered as
//  smin metaballs in the fragment shader. EVERYTHING below is the tuning surface for
//  the lava look; Daniel reads + tunes these.
//
//  Coordinate convention (shared with the shader): physics runs in HEIGHT-NORMALIZED
//  space — every position/velocity/radius is in units of (pixel / canvasHeight). So
//  y ∈ [0, 1] top→bottom, x ∈ [0, W/H], and the FLOOR (footer / lava rest line) is the
//  bottom edge of the canvas at y = 1 (the canvas is already CSS-clipped to the footer
//  top in R1, so its own bottom edge IS the footer line — no extra clip uniform needed).
//  Using one isotropic unit keeps blobs round and collisions correct at any aspect.
// ════════════════════════════════════════════════════════════════════════════════════

/**
 * Hard upper bound on the simulated/rendered blob population. The per-fragment shader
 * loops to this constant, so it caps GPU cost; the CPU step is O(MAX_BLOBS²) for
 * blob↔blob (≤ ~1k pair tests — trivial). 32 is the spec's upper band (§4g). The live
 * count varies with the density dial but never exceeds this.
 */
const MAX_BLOBS = 32;

/** Lower end of the live blob count (density dial 0 → a few large lazy blobs, §4e). */
const MIN_BLOB_COUNT = 16;

/** Blob radius band in height-normalized units. 0.025·H ≈ 20px and 0.13·H ≈ 100px on a
 *  ~760px-tall canvas — the spec's ~20–100px range (§4b). Each blob picks a fixed radius
 *  in this band at construction; the density dial biases the average. */
const BLOB_RADIUS_MIN = 0.025;
const BLOB_RADIUS_MAX = 0.13;

/**
 * Gravity acceleration at the gravity dial = 1, in height-units / s². Downward (+y).
 * Tuned so wax at full gravity falls back to the floor in well under a second from
 * mid-screen (0.5 height) — a firm settle — while still letting buoyancy win when hot.
 * The dial scales this 0→1 linearly (dial 0 = near-weightless float, §4d).
 */
const GRAVITY_ACCEL_MAX = 2.2;
/** Floor of gravity even at dial 0, so wax never becomes truly weightless (always settles). */
const GRAVITY_ACCEL_MIN = 0.15;

/**
 * Buoyancy lift coefficient: upward accel = BUOYANCY_COEFF · heatScale · (T − T_ambient).
 * Hot wax (T high) rises; cool wax (T low) sinks. This is the OTHER half of the lamp's
 * core tension against gravity. Tuned against GRAVITY_ACCEL_MAX so that at full heat a
 * hot blob (T≈1) overcomes mid-gravity and climbs, and at heat 0 (heatScale 0) buoyancy
 * vanishes entirely → wax just obeys gravity and rests on the floor (spec §4c endpoint).
 */
const BUOYANCY_COEFF = 4.0;
const TEMP_AMBIENT = 0.5; // the neutral temperature; above it lifts, below it sinks

/**
 * Heat transfer rates (per second), the engine of the convection cycle:
 *   - near the FLOOR a blob HEATS toward 1 (the "lamp bulb" at the bottom),
 *   - near the TOP it COOLS toward 0 (loses heat at the cold cap),
 *   - everywhere it relaxes gently toward ambient.
 * heatScale (the heat dial's transfer-function output, see heatScaleFromDial) gates the
 * floor-heating rate: at dial 0 the floor adds NO heat, so nothing ever becomes buoyant
 * and the pool rests; at dial 1 the floor pumps heat fast → many blobs go buoyant and
 * rise per second (the busy roiling lamp, §4c max endpoint).
 */
const HEAT_FLOOR_RATE = 1.6;   // °/s toward T=1 when sitting on the floor (× heatScale)
const HEAT_TOP_RATE = 1.2;     // °/s toward T=0 when near the top
const HEAT_AMBIENT_RATE = 0.25; // °/s relaxation toward ambient everywhere
const HEAT_FLOOR_ZONE = 0.16;  // height-fraction above the floor counted as "hot zone"
const HEAT_TOP_ZONE = 0.16;    // height-fraction below the top counted as "cold zone"

/**
 * Viscous (linear) velocity damping per second — the lazy/high-viscosity regime that
 * makes it read as wax, not water (spec §4a). Applied as v *= exp(−DAMPING·dt) each
 * step, so it is frame-rate independent. High enough that motion is slow and gooey;
 * low enough that hot blobs still make the trip up.
 */
const VISCOUS_DAMPING = 1.4;

/**
 * Soft floor contact: instead of a hard clamp that jitters, a resting blob is pushed up
 * by a spring proportional to its penetration below the floor, and its downward velocity
 * is killed on contact so pooled wax flattens and settles rather than bouncing forever.
 */
const FLOOR_SPRING = 26.0;      // restoring accel per unit penetration (height-units/s²)
const FLOOR_CONTACT_DAMPING = 6.0; // extra damping applied while in floor contact (settle)

/**
 * Blob↔blob collision: the soft↔hard knob (collision strength) blends a penalty SPRING
 * (soft displacement, blobs squish and partially overlap then ease apart) toward an
 * elastic IMPULSE (hard, crisp restitution along the centre line). These are the two
 * endpoints the strength dial interpolates (§5c). Restitution is the bounciness of the
 * hard end; the spring stiffness is the firmness of the soft end.
 */
const BLOB_COLLIDE_SPRING = 14.0;    // soft penalty stiffness (height-units/s² per overlap)
const BLOB_RESTITUTION_HARD = 0.9;   // elastic restitution at strength = 1 (near-perfect bounce)
const BLOB_RESTITUTION_SOFT = 0.15;  // residual restitution at strength = 0 (mostly absorptive)

/**
 * Blob↔waveform collision (always on, independent of heat — §5b). The waveform's
 * half-width at a blob's row is sampled CPU-side each frame; a blob whose centre is
 * within (halfWidth + radius) of the centre line is penetrating the ribbon and is pushed
 * out along the surface normal. Same soft↔hard blend as blob↔blob: a penalty spring at
 * the soft end → elastic reflection of the inward velocity at the hard end. The waveform
 * is read-only authority: it pushes the fluid, the fluid never moves it.
 */
const WAVE_COLLIDE_SPRING = 20.0;    // soft penalty stiffness pushing wax off the ribbon
const WAVE_RESTITUTION_HARD = 0.85;  // elastic reflection strength at full collision hardness
const WAVE_RESTITUTION_SOFT = 0.1;

/**
 * Max physics timestep, seconds. rAF can stall (tab blur, GC); a huge dt would let a
 * blob tunnel through the floor or another blob in one step (explosive overlap). We clamp
 * dt so the integrator stays stable — a long stall just means the sim advances a little
 * slowly that frame, which is invisible. (We also sub-step within this cap below.)
 */
const PHYSICS_MAX_DT = 1 / 30;
/** Sub-steps per frame: splitting dt makes the spring/penalty collisions stiffer-stable. */
const PHYSICS_SUBSTEPS = 2;

/**
 * Playhead-correction smoothing time constant, in seconds. Governs how fast the
 * rendered playhead absorbs a re-anchor discontinuity at each ~10 Hz push.
 *
 * The problem: the player's position reports are irregular at startup (buffering /
 * playback ramp-up), so each push lands a position that doesn't match where the
 * wall-clock interpolation had advanced to. Hard-anchoring to each push (the prior
 * behaviour) made that gap a visible snap every push — the startup jitter.
 *
 * The fix (classic netcode-style entity reconciliation): the player stays the sole
 * source of truth, but instead of rendering the authoritative position directly, we
 * render authoritative + a small *correction offset* that decays toward zero every
 * frame. On each push we fold the re-anchor discontinuity into that offset so the
 * rendered playhead is continuous across the push, then bleed the offset off over
 * ~this time constant. This eases the snap into a sub-perceptible glide.
 *
 * Why an offset that decays to zero, not an absolute lerp toward target: a lerp
 * toward the target leaves a steady-state lag proportional to velocity (the render
 * always trailing real playback). Decaying the *error* to zero converges the
 * rendered playhead back onto the authoritative one, so once pushes steady the
 * offset is ~0 and behaviour is identical to the old hard-anchor — no lag, and
 * steady-state is unchanged as required.
 *
 * 0.12 s is a sensible default: long enough to dissolve the worst startup snaps
 * (tens of ms of position error), short enough that the correction is imperceptible
 * and the render never trails real playback by more than a few ms. Tunable.
 */
const PLAYHEAD_CORRECTION_TIME_CONSTANT_SECONDS = 0.12;

/**
 * Below this absolute correction (seconds) we snap the offset to 0 and stop easing —
 * an exponential decay never mathematically reaches zero, and carrying a sub-ms
 * residual forever is pointless. ~0.5 ms is well under one frame of motion at any
 * real zoom, so collapsing it is invisible.
 */
const PLAYHEAD_CORRECTION_SNAP_SECONDS = 0.0005;

// ── Diagnostics ──────────────────────────────────────────────────────────────────
//
// Set true to surface the init/draw/datum/playback seams to the browser console
// (all prefixed `[MixVisualizer]`). The error/warn paths fire regardless of this
// flag — they only trigger on the abnormal path. The verbose `log` paths (datum
// received/uploaded, first-draw dimensions, GL error after first draw) are gated
// here so they can be silenced once the renderer is confirmed healthy. Leave it on
// while the runtime fix is being verified through the browser.
// NOTE: ON for this visual-iteration pass (Phase 10 W3 rework). Daniel tests in-browser;
// the resolved navy/moss RGB + FPS lines confirm the fixes. Flip back to false once the
// look is approved.
const DEBUG = true;

const TAG = '[MixVisualizer]';
function debugLog(...args: unknown[]): void {
    if (DEBUG) console.log(TAG, ...args);
}

// ── Theme: the navy↔moss field poles, read live from the active MudBlazor palette. ─
//
// The shader cannot resolve `var(--mud-palette-*)` directly — uniforms are plain
// floats. So we read the computed `--mud-palette-*` custom properties straight off
// the canvas element (which inherits them from the page), parse them to RGB, and
// upload them as vec3 colour uniforms. The bespoke light/dark themes swap those vars
// when dark mode toggles, so re-reading + re-uploading them re-themes the field with
// no reload. The component just calls `refreshTheme()` after a dark-mode change.
//
// Wave 3 binding — the two poles of the morphing colour field (spec §4b/§4c):
//   - `uColorAccent` carries MOSS (the interactive green).
//   - `uColorEdge`   carries NAVY (the dark ground / navy-mid).
// (The names are inherited from the parity two-stop gradient; in Wave 3 they are the
// two field poles, not a now-line→edge luminance ramp. Kept rather than renamed to
// avoid touching the bridge's uniform-location cache and the well-tested upload path.)
//
// The cross-mode problem (spec §4c, explicit): navy and moss are NOT a single stable
// pair of CSS vars across both palettes. Navy is `--mud-palette-primary` in LIGHT but
// the `--mud-palette-background` ground in DARK; moss is `--mud-palette-secondary` in
// LIGHT but `--mud-palette-primary` in DARK (where green IS primary). No one var holds
// "navy" or "moss" in both modes. So we detect the mode in JS (by the luminance of the
// page background — the bespoke dark ground #0D1B2A is near-black, the light ground
// #FAFAF8 is near-white) and bind the poles per the spec's stated mapping. refreshTheme
// re-runs this on a dark-mode toggle, so the field re-themes live.

interface ResolvedTheme {
    /** Moss-green pole RGB [0,1] — uploaded to uColorAccent. */
    accent: [number, number, number];
    /** Navy pole RGB [0,1] — uploaded to uColorEdge. */
    edge: [number, number, number];
}

/** sRGB relative luminance (cheap Rec.709 weights) of a normalized RGB triple. */
function luminance([r, g, b]: [number, number, number]): number {
    return 0.2126 * r + 0.7152 * g + 0.0722 * b;
}

/**
 * Read a CSS custom property off an element, falling back if it is empty/undefined.
 * An empty value means the var did not inherit onto the canvas (e.g. the palette is
 * scoped to a wrapper the canvas isn't under), which would silently swap the ribbon
 * colour for the hardcoded default — so warn on it when diagnosing.
 */
function readVar(el: Element, name: string, fallback: string): string {
    const v = getComputedStyle(el).getPropertyValue(name).trim();
    if (v.length === 0) {
        if (DEBUG) console.warn(`${TAG} CSS var '${name}' did not resolve off the canvas — using fallback '${fallback}'; ribbon colour may be wrong.`);
        return fallback;
    }
    return v;
}

/**
 * Parse a CSS colour string to normalized [0,1] RGB. Handles #rgb / #rrggbb and
 * rgb()/rgba() — the only forms MudBlazor emits for these palette vars. Falls back
 * to mid-grey on anything unrecognised so a parse miss degrades to a visible
 * ribbon rather than black.
 */
function parseColor(css: string): [number, number, number] {
    const s = css.trim();
    if (s.startsWith('#')) {
        let hex = s.slice(1);
        if (hex.length === 3) {
            hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
        }
        if (hex.length >= 6) {
            const r = parseInt(hex.slice(0, 2), 16);
            const g = parseInt(hex.slice(2, 4), 16);
            const b = parseInt(hex.slice(4, 6), 16);
            if (!Number.isNaN(r) && !Number.isNaN(g) && !Number.isNaN(b)) {
                return [r / 255, g / 255, b / 255];
            }
        }
    }
    const m = s.match(/rgba?\(\s*([\d.]+)\s*,\s*([\d.]+)\s*,\s*([\d.]+)/i);
    if (m) {
        return [Number(m[1]) / 255, Number(m[2]) / 255, Number(m[3]) / 255];
    }
    return [0.5, 0.5, 0.5];
}

// ── Datum: the pre-downloaded loudness profile (spec §F). ────────────────────────

interface Datum {
    /**
     * GPU texture holding the loudness samples (R8). Laid out as a 2-D grid that
     * respects GL_MAX_TEXTURE_SIZE (see uploadDatum) rather than a 1×N row, which
     * blows past the max texture width for any mix over ~49 s at the ~333 samples/s
     * datum density. The shader reads it with texelFetch (integer addressing), so no
     * hardware filtering is used — see sampleAt for the manual interpolation.
     */
    texture: WebGLTexture;
    /** Texture width in texels (samples per row). */
    texWidth: number;
    /** Texture height in texels (number of rows). */
    texHeight: number;
    /** Number of real samples in the datum (≤ texWidth*texHeight; the tail row is padded). */
    sampleCount: number;
    /** Total mix duration in seconds — needed to map time <-> sample index. */
    durationSeconds: number;
    /**
     * The decoded loudness bytes [0,255], retained for CPU-side sampling by the physics
     * step (the waveform-collision boundary is sampled per blob per frame — R2 §5). The
     * GPU has its own copy in `texture`; this is the CPU mirror, kept because re-reading
     * the texture back from the GPU each frame would be a stall.
     */
    samples: Uint8Array;
}

interface Playback {
    /**
     * Last playback head pushed from Blazor, in seconds. This is the *authoritative*
     * position the player last reported — it updates only on the ~10 Hz setPlayback
     * push, NOT every frame. The per-frame scroll uses the interpolated
     * effectivePlayhead (see draw()), anchored on this value.
     */
    positionSeconds: number;
    /** Whether audio is actively playing — gates the rAF loop so a paused mix stays cool. */
    isPlaying: boolean;
    /**
     * performance.now() (ms) captured when positionSeconds was pushed. The rAF loop
     * advances the playhead by wall-clock elapsed since this anchor so the ribbon
     * scrolls smoothly at the display refresh rate between the sparse ~10 Hz pushes,
     * instead of stepping once per push (the ~10 FPS smoothness bug). Re-anchored on
     * every push, so each push is a small correction rather than a hard reset.
     */
    pushWallClockMs: number;
}

export interface MixVisualizerHandle {
    setDatum(samplesBase64: string, durationSeconds: number): void;
    setPlayback(positionSeconds: number, isPlaying: boolean): void;
    setZoom(visibleSeconds: number): void;
    /** [0,1]. R2 TEMP: routes the "Bubblyness" knob to lava GRAVITY (R4 renames). */
    setBubblyness(value: number): void;
    /** [0,1]. R2 TEMP: routes the "Detach" knob to lava HEAT (R4 renames). */
    setDetach(value: number): void;
    /** [0,1]. R2 TEMP: routes the "Color-shift" knob to COLLISION STRENGTH (R4 renames). */
    setColorShiftSpeed(value: number): void;
    /** Re-read the palette CSS vars off the canvas (call after a dark-mode toggle). */
    refreshTheme(): void;
    dispose(): void;
}

/**
 * Decode the base64 loudness datum (bytes [0,255]) into a Uint8Array suitable for
 * direct upload as an R8 texture. Done once per datum, off the animation path. We
 * keep the bytes as [0,255] and let the GPU normalize to [0,1] on sample (R8
 * UNORM), which mirrors the predecessor's /255 and avoids a CPU float pass.
 */
function decodeSamples(base64: string): Uint8Array {
    const binary = atob(base64);
    const out = new Uint8Array(binary.length);
    for (let i = 0; i < binary.length; i++) {
        out[i] = binary.charCodeAt(i);
    }
    return out;
}

// ── Shaders. ─────────────────────────────────────────────────────────────────────
//
// Vertex: trivial pass-through. We draw a single triangle that more than covers the
// clip-space box ([-1,1]²) so every pixel of the canvas is rasterized once. (One
// oversized triangle is the standard full-screen trick — cheaper than two and has
// no diagonal seam.) gl_FragCoord in the fragment shader then gives us the pixel's
// screen position directly.

const VERTEX_SHADER = `#version 300 es
// Three clip-space vertices forming one big triangle covering [-1,1]².
const vec2 POSITIONS[3] = vec2[3](
    vec2(-1.0, -1.0),
    vec2( 3.0, -1.0),
    vec2(-1.0,  3.0)
);
void main() {
    gl_Position = vec4(POSITIONS[gl_VertexID], 0.0, 1.0);
}
`;

// Fragment: THE SCROLL + ZOOM MATH (spec §A, §B), ported intact from the Canvas
// predecessor's per-row loop into a per-fragment evaluation. Read this top to
// bottom to follow how a quarter-note-@-180-BPM becomes 0.333 s becomes a texture
// coordinate becomes a lit pixel.
//
// Coordinate model (matches the Canvas predecessor exactly):
//   - gl_FragCoord.xy is in device pixels, origin BOTTOM-left in WebGL. The Canvas
//     used a TOP-left origin with Y increasing downward. We flip Y once up front
//     (screenYTop) so all the time math below reads identically to the Canvas
//     version: screenYTop = 0 at the top edge, = uResolution.y at the bottom.
//   - The "now" line is a fixed screen Y: nowY = height * NOW_ANCHOR_FROM_TOP.
//   - Audio flows UP: newer audio is drawn lower and scrolls up past the now line.
//       * audio BELOW the now line (screenYTop > nowY) is the lead-in (not yet played)
//       * audio ABOVE the now line (screenYTop < nowY) is the trail-out (just played)
//
// Zoom -> time-span -> pixels:
//   - uVisibleSeconds is the whole window's time span, top to bottom. At max zoom
//     this is MIN_VISIBLE_SECONDS (0.333 s); at min zoom MAX_VISIBLE_SECONDS.
//   - pixelsPerSecond = height / uVisibleSeconds. Smaller visibleSeconds => more px
//     per second => the same audio sweeps the window faster at a fixed playback
//     rate. That IS the Guitar-Hero coupling: apparent scroll speed falls straight
//     out of the zoom, with no separate speed control.
//
// Time at a given screen Y:
//   - At nowY the time is uPlayheadSeconds.
//   - Moving DOWN by 1 px (screenYTop +1) adds (1 / pixelsPerSecond) seconds.
//   - So:  timeAt(y) = playhead + (screenYTop - nowY) / pixelsPerSecond
//
// Sample at a given time:
//   - time / durationSeconds is the normalized position along the mix; multiplied by
//     the sample count it becomes a continuous sample index. sampleAt interpolates
//     between the two bracketing samples by hand (texelFetch + fract lerp) — see the
//     note on its definition for WHY we can't use hardware LINEAR with the 2-D layout.
//   - Outside [0, durationSeconds] we force loudness to 0. That is what gives the
//     "scrolls in from empty / out to empty" behaviour at the very start and end of
//     the mix (spec §A) with no special-casing. (CLAMP_TO_EDGE on the texture would
//     otherwise repeat the edge sample, so we gate explicitly here.)

const FRAGMENT_SHADER = `#version 300 es
precision highp float;

uniform vec2  uResolution;       // canvas size in device pixels
uniform float uPlayheadSeconds;  // current playback position (per-frame)
uniform float uTimeSeconds;      // monotonic clock (per-frame) — drives field morph
uniform float uVisibleSeconds;   // zoom: window time-span (per change)
// NOTE: the lava physics params (gravity/heat/collision/density) are NOT shader uniforms
// in R2 — they drive the CPU physics step, which uploads the resulting uBlobs[]. The old
// uBubblyness/uDetach/uColorShiftSpeed uniforms are gone from the shader for that reason;
// the JS handle still receives those control values and routes them to the physics (the
// R2 TEMP knob re-mapping documented at the control-default consts above).
uniform float uDurationSeconds;  // mix length (per datum)
uniform vec3  uColorAccent;      // MOSS pole of the field (per theme)
uniform vec3  uColorEdge;        // NAVY pole of the field (per theme)
uniform float uHasDatum;         // 1.0 when a datum texture is bound, else 0.0
uniform sampler2D uDatum;        // loudness profile, R8, 2-D grid, NEAREST (texelFetch)
uniform int   uDatumWidth;       // datum texture width in texels (samples per row)
uniform int   uDatumSampleCount; // number of real samples (tail row is padded)

// ── R2 wax-blob uniforms (the CPU physics step uploads these every frame). ──────────
// Each blob is packed as a vec4: xy = centre in HEIGHT-NORMALIZED space (pixel/H, so
// y is 0 at the top edge and 1 at the footer/floor, x spans [0, W/H]); z = radius in
// the SAME height-normalized unit (so circles are round on screen); w = temperature
// 0..1 (drives the warm tint on hot rising wax). uBlobCount is how many of the
// MAX_BLOBS slots are live this frame. Working in height-normalized units keeps the
// metaball SDF isotropic regardless of the canvas aspect ratio.
const int MAX_BLOBS = ${MAX_BLOBS};
uniform vec4 uBlobs[MAX_BLOBS];
uniform int  uBlobCount;

out vec4 fragColor;

const float NOW_ANCHOR_FROM_TOP = ${NOW_ANCHOR_FROM_TOP.toFixed(4)};
const float RIBBON_HALF_WIDTH_FRAC = ${RIBBON_HALF_WIDTH_FRAC.toFixed(4)};

// ── R2 in-shader tuning constants (Daniel tunes by editing here). ───────────────────

// Background opacity of the wax + waveform fill. Kept simple/serviceable for R2 — the
// beautiful OKLab three-colour gradient is Wave R3. Just enough to read the physics.
const float RIBBON_OPACITY_R2 = 0.62;

// smin blend radius for the wax metaball union, in height-normalized units. Larger = the
// "necks" where two blobs merge are fatter → a gooier, more-connected wax that splits and
// recombines (the organic non-circular look the spec wants, §4b). This + varied radii are
// what kill the "giant disconnected circles" failure.
const float BLOB_SMOOTHMIN_K = 0.045;

// smin blend radius for merging the wax into the WAVEFORM ribbon, so resting/pooled wax
// reads as continuous with the ribbon surface rather than a disc sitting on a wall.
const float WAVE_SMOOTHMIN_K = 0.03;

// Low-frequency, blob-tied radius wobble: a slow per-blob breathing so each wax shape is
// organic, not a perfect circle (§4b). This is FLUID-tied noise (keyed to blob identity +
// the wall clock), NOT the screen-space "dirt" R1 removed (§3) — it travels with the wax.
const float BLOB_WOBBLE_AMOUNT = 0.12; // ± fraction of radius
const float BLOB_WOBBLE_RATE   = 0.7;  // breathing speed (rad/s scale)

// Warm tint on hot, rising wax. A hot blob (temperature → 1) shifts slightly toward a
// warm highlight so the eye reads "this one is rising"; cool wax stays the cool field
// colour. Serviceable placeholder until R3's real colour model — kept subtle.
const vec3  HOT_TINT        = vec3(0.95, 0.72, 0.45); // warm amber the hottest wax leans toward
const float HOT_TINT_AMOUNT = 0.35; // max lean at temperature 1 (above ambient)

// Fetch one raw sample by its linear index, mapping the 1-D index onto the 2-D
// texture grid (col = i mod width, row = i / width). texelFetch ignores filtering
// and wrap modes — it reads the exact texel — so the row-wrap layout is invisible
// to the caller.
float fetchSample(int i) {
    int col = i % uDatumWidth;
    int row = i / uDatumWidth;
    return texelFetch(uDatum, ivec2(col, row), 0).r;
}

// Loudness at an absolute mix time, or 0 outside the mix (drives scroll-in/out).
//
// Interpolation note: we cannot lean on hardware LINEAR filtering here. The datum
// is laid across a 2-D grid (1×N would exceed GL_MAX_TEXTURE_SIZE past ~49 s of
// mix), and a hardware 2D-LINEAR read would blend across the row-wrap seam at the
// end of every row — sample[width-1] would wrongly bleed into sample[width] of the
// next row, and bilinear would also pull in the row above/below. So we do the
// linear interpolation by hand along the TIME axis only: bracket the fractional
// sample position with the two neighbouring texels, texelFetch each (each correctly
// mapped to its own 2-D texel), and lerp. Exact, no seam artifact.
//
// Texel-centre convention: this reproduces the predecessor's 1-D LINEAR read bit for
// bit. There, u = t/duration sampled an N-texel LINEAR texture, whose texel centres
// sit at (i+0.5)/N — so u maps to texel-space position u*N - 0.5, interpolating
// between floor() and floor()+1 of that, with CLAMP_TO_EDGE at the ends. We mirror
// exactly that here: the -0.5 and the index clamps to [0, N-1] are the CLAMP_TO_EDGE
// behaviour at both extremes.
float sampleAt(float timeSeconds) {
    if (uHasDatum < 0.5) return 0.0;
    if (timeSeconds < 0.0 || timeSeconds >= uDurationSeconds) return 0.0;
    float n = float(uDatumSampleCount);
    // Continuous texel-space position, half-texel shifted to match LINEAR centres.
    float p = (timeSeconds / uDurationSeconds) * n - 0.5;
    int i0 = clamp(int(floor(p)),     0, uDatumSampleCount - 1);
    int i1 = clamp(int(floor(p)) + 1, 0, uDatumSampleCount - 1);
    float f = clamp(p - floor(p), 0.0, 1.0);
    return mix(fetchSample(i0), fetchSample(i1), f);
}

// ════════════════════════════════════════════════════════════════════════════════════
//  R2 — wax metaballs + waveform SDF. The blobs are integrated on the CPU (see the JS
//  physics step) and uploaded as uBlobs[]; the shader composites them with smin and the
//  waveform ribbon into one liquid surface, then shades it with a simple theme fill.
// ════════════════════════════════════════════════════════════════════════════════════

// ── Value-noise (used now only for the organic, blob-tied radius wobble). ────────────
// A standard hash → smooth value-noise. Cheap (a few mixes), no texture lookup, and
// continuous. Fed blob-identity + the wall clock it gives each wax shape its own slow
// breathing so the silhouette is organic rather than a perfect circle (§4b).
float hash21(vec2 p) {
    p = fract(p * vec2(123.34, 345.45));
    p += dot(p, p + 34.345);
    return fract(p.x * p.y);
}
float valueNoise(vec2 p) {
    vec2 i = floor(p);
    vec2 f = fract(p);
    // Smoothstep (Hermite) interpolation of the four lattice corners — C1-continuous.
    vec2 u = f * f * (3.0 - 2.0 * f);
    float a = hash21(i + vec2(0.0, 0.0));
    float b = hash21(i + vec2(1.0, 0.0));
    float c = hash21(i + vec2(0.0, 1.0));
    float d = hash21(i + vec2(1.0, 1.0));
    return mix(mix(a, b, u.x), mix(c, d, u.x), u.y);
}

// ── Signed-distance primitives + smooth-min (the metaball machinery). ───────────────
// Circle SDF (a metaball centre) — the wax blob primitive.
float sdCircle(vec2 p, float r) {
    return length(p) - r;
}
// Polynomial smooth-min (Inigo Quilez). Blends two SDFs into one continuous surface —
// the metaball union. k controls the blend radius (the size of the liquid "neck" where
// two blobs merge). As k→0 it becomes a hard min (discrete shapes); larger k fuses them.
float smin(float a, float b, float k) {
    float h = clamp(0.5 + 0.5 * (b - a) / k, 0.0, 1.0);
    return mix(b, a, h) - k * h * (1.0 - h);
}

// ── The waveform ribbon SDF, in HEIGHT-NORMALIZED space (negative inside). ──────────
//
// The waveform is the same symmetric ±loudness ribbon about the centre line as before,
// but evaluated in height-normalized coords (pixel/H) so it shares one space with the
// wax blobs. p = (x, y) where x ∈ [0, W/H] across the canvas and y ∈ [0, 1] top→bottom.
// We map the row's mix-time → loudness → a half-width about the centre x, and return the
// distance to that vertical ribbon band. Loudness at neighbour rows is NOT re-stacked
// here (the per-row geometry from Wave 1 is already smooth); the band is the ribbon.
float waveformSdf(vec2 p, float aspect, float nowYn, float secondsPerHeight) {
    // Mix-time at this row: rows below the now-line are future audio, above are past.
    float t = uPlayheadSeconds + (p.y - nowYn) * secondsPerHeight;
    float amp = sampleAt(t);                         // loudness 0..1 at this row
    float centreX = aspect * 0.5;                    // canvas centre x in height-norm units
    float halfW = amp * (aspect * 0.5) * RIBBON_HALF_WIDTH_FRAC; // ribbon half-width here
    // Distance to a centred vertical band of half-width halfW: |x − centre| − halfW.
    // Negative inside the band, positive outside. (A pure horizontal band; the vertical
    // extent is the whole column, which is what the scrolling ribbon is.)
    return abs(p.x - centreX) - halfW;
}

// ── The combined wax + waveform liquid SDF at a height-normalized point. ─────────────
//
// Unions all live wax blobs (smin metaballs) and the waveform ribbon into one continuous
// surface. The blob radii carry a slow blob-tied wobble so each is organic, not a perfect
// circle. Returns the signed distance and, via out params, the nearest-blob temperature
// (for the warm hot-wax tint) and whether the point is dominated by wax vs. ribbon.
float liquidSdf(vec2 p, float aspect, float nowYn, float secondsPerHeight,
                out float hotOut) {
    // Waveform ribbon first — the always-present base surface.
    float field = waveformSdf(p, aspect, nowYn, secondsPerHeight);

    float hotAccum = 0.0;
    float hotWeight = 0.0;

    // Union every live wax blob. Bounded loop to MAX_BLOBS; uBlobCount gates the live set.
    for (int i = 0; i < MAX_BLOBS; i++) {
        if (i >= uBlobCount) break;
        vec4 b = uBlobs[i];
        vec2 c = b.xy;       // centre, height-norm
        float r = b.z;       // radius, height-norm
        float temp = b.w;    // temperature 0..1

        // Organic radius wobble: a slow per-blob breathing (blob-tied + wall clock), so
        // the silhouette is never a clean circle. Fluid-tied, not screen-space (§3 ok).
        float wob = (valueNoise(vec2(float(i) * 1.37, uTimeSeconds * BLOB_WOBBLE_RATE)) - 0.5)
                    * 2.0 * BLOB_WOBBLE_AMOUNT;
        float rr = r * (1.0 + wob);

        float blob = sdCircle(p - c, rr);
        field = smin(field, blob, BLOB_SMOOTHMIN_K);

        // Weight this blob's temperature by proximity so the tint follows the nearest wax.
        float prox = clamp(1.0 - (blob / max(rr, 1e-3)), 0.0, 1.0);
        hotAccum += temp * prox;
        hotWeight += prox;
    }

    hotOut = hotWeight > 1e-3 ? hotAccum / hotWeight : 0.0;
    return field;
}

void main() {
    float w = uResolution.x;
    float h = uResolution.y;

    // Empty backdrop when there is no datum (no thin-centre-line artifact — Wave 1 note).
    if (uHasDatum < 0.5) {
        fragColor = vec4(0.0);
        return;
    }

    // Height-normalized fragment coordinate (pixel / H), top-left origin, y down. This is
    // the shared space the CPU physics works in — the blob uniforms are already in it.
    float aspect = w / h;                       // canvas width in height units
    vec2 p = vec2(gl_FragCoord.x / h, (h - gl_FragCoord.y) / h);

    float nowYn = NOW_ANCHOR_FROM_TOP;          // now-line, height-norm (y ∈ [0,1])
    float secondsPerHeight = uVisibleSeconds;   // one full height spans uVisibleSeconds

    // ── Evaluate the combined liquid SDF + its gradient (the surface normal). ──────────
    // Central differences in height-norm space; the step is one device pixel = 1/h.
    float hot;
    float d = liquidSdf(p, aspect, nowYn, secondsPerHeight, hot);

    float e = 1.0 / h; // one-pixel step in height-norm units
    float ig;
    float dRx = liquidSdf(p + vec2(e, 0.0), aspect, nowYn, secondsPerHeight, ig);
    float dLx = liquidSdf(p - vec2(e, 0.0), aspect, nowYn, secondsPerHeight, ig);
    float dDy = liquidSdf(p + vec2(0.0, e), aspect, nowYn, secondsPerHeight, ig);
    float dUy = liquidSdf(p - vec2(0.0, e), aspect, nowYn, secondsPerHeight, ig);
    vec2 grad = vec2(dRx - dLx, dDy - dUy);
    vec2 normal = length(grad) > 1e-5 ? normalize(grad) : vec2(0.0, -1.0);

    // Inside-ness: SDF negative = inside. Feather ~1.2px (in height-norm units) for an
    // anti-aliased edge instead of a hard chart line (no blur — spec §2/§3).
    float pxFeather = 1.2 / h;
    float inside = 1.0 - smoothstep(-pxFeather, pxFeather, d);
    if (inside <= 0.0) { fragColor = vec4(0.0); return; }

    // ── Simple serviceable theme fill (R3 replaces with the OKLab three-colour gradient).
    // Linear A→B from the centre line outward: NAVY (uColorEdge) at the root, MOSS
    // (uColorAccent) at the extended edge. Just enough colour to read the physics; NOT the
    // final colour model. No HSL, no vivify, no glass — those are gone (R3 owns colour).
    float xnAbs = clamp(abs(p.x - aspect * 0.5) / (aspect * 0.5), 0.0, 1.0);
    vec3 fill = mix(uColorEdge, uColorAccent, xnAbs);

    // Warm tint on hot, rising wax so the eye reads convection (serviceable, R3-subordinate).
    float hotLean = clamp((hot - ${TEMP_AMBIENT.toFixed(2)}) * 2.0, 0.0, 1.0) * HOT_TINT_AMOUNT;
    fill = mix(fill, HOT_TINT, hotLean);

    // A soft top-light shade off the surface normal so the wax has form (a single lazy
    // gradient, not the old four-part glass). Keeps it from reading flat without competing
    // with the (future) colour model.
    float lightUp = clamp(dot(normal, vec2(0.0, -1.0)) * 0.5 + 0.5, 0.0, 1.0);
    fill *= mix(0.82, 1.12, lightUp);

    float alpha = inside * RIBBON_OPACITY_R2;
    fragColor = vec4(fill * alpha, alpha); // pre-multiplied for ONE/ONE_MINUS_SRC_ALPHA
}
`;

/** Compile one shader stage, throwing with the info log on failure. */
function compileShader(gl: WebGL2RenderingContext, type: number, source: string): WebGLShader {
    const shader = gl.createShader(type);
    if (!shader) throw new Error('MixVisualizer: gl.createShader returned null.');
    gl.shaderSource(shader, source);
    gl.compileShader(shader);
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        const log = gl.getShaderInfoLog(shader);
        gl.deleteShader(shader);
        throw new Error(`MixVisualizer: shader compile failed: ${log}`);
    }
    return shader;
}

/** Link the vertex + fragment shaders into a program, throwing on failure. */
function linkProgram(gl: WebGL2RenderingContext): WebGLProgram {
    const vert = compileShader(gl, gl.VERTEX_SHADER, VERTEX_SHADER);
    const frag = compileShader(gl, gl.FRAGMENT_SHADER, FRAGMENT_SHADER);
    const program = gl.createProgram();
    if (!program) throw new Error('MixVisualizer: gl.createProgram returned null.');
    gl.attachShader(program, vert);
    gl.attachShader(program, frag);
    gl.linkProgram(program);
    // Shaders can be deleted after link — the program retains the compiled code.
    gl.deleteShader(vert);
    gl.deleteShader(frag);
    if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
        const log = gl.getProgramInfoLog(program);
        gl.deleteProgram(program);
        throw new Error(`MixVisualizer: program link failed: ${log}`);
    }
    return program;
}

/** The no-op handle returned when WebGL2 is unavailable or setup fails. */
function noopHandle(): MixVisualizerHandle {
    return {
        setDatum() {},
        setPlayback() {},
        setZoom() {},
        setBubblyness() {},
        setDetach() {},
        setColorShiftSpeed() {},
        refreshTheme() {},
        dispose() {},
    };
}

export function create(canvas: HTMLCanvasElement): MixVisualizerHandle {
    // premultipliedAlpha so the translucent ribbon composites correctly over the
    // page; antialias off (the soft-edge smoothstep handles AA in-shader, and MSAA
    // would cost fill rate we don't need for a backdrop).
    const maybeGl = canvas.getContext('webgl2', {
        alpha: true,
        premultipliedAlpha: true,
        antialias: false,
    });
    if (!maybeGl) {
        // No WebGL2 (old engine / disabled): hand back a no-op handle so the
        // component still functions as a plain backdrop (mirrors the predecessor's
        // no-2d-context fallback, now guarding against no-WebGL2).
        console.error(`${TAG} getContext('webgl2') returned null — WebGL2 unavailable; rendering a plain backdrop.`);
        return noopHandle();
    }
    // Non-null binding so the closures below keep the narrowing (TS does not carry
    // control-flow narrowing of a captured `const` into nested functions).
    const gl: WebGL2RenderingContext = maybeGl;

    // GL_MAX_TEXTURE_SIZE is a per-context constant — query it once. The datum is
    // laid out across a 2-D grid no wider than this (see uploadDatum); a 1×N row
    // would exceed it for any mix over ~49 s at the ~333 samples/s datum density,
    // and texImage2D would reject the upload (the bug this fix addresses).
    const maxTextureSize: number = gl.getParameter(gl.MAX_TEXTURE_SIZE) as number;

    let program: WebGLProgram;
    try {
        program = linkProgram(gl);
    } catch (err) {
        // A compile/link failure on an exotic driver should degrade to the plain
        // backdrop, not crash the page. Log for diagnosis; return the no-op handle.
        console.error(`${TAG} shader compile/link failed; rendering a plain backdrop.`, err);
        return noopHandle();
    }

    // An empty VAO is still required in WebGL2 core to issue a draw; the vertex
    // shader sources its positions from gl_VertexID, so no attribute buffers.
    const vao = gl.createVertexArray();

    // Cache uniform locations once. A null here for a uniform we actually upload
    // means either the name is misspelled or the GLSL compiler dead-stripped it
    // (it isn't reachable in the shader) — both of which silently break a uniform's
    // effect, so surface them. No reserved-unused exemptions remain: every uniform
    // below is genuinely consumed by the R2 shader (the old inert Wave-3 control
    // uniforms are gone — the lava params drive the CPU physics, not the shader).
    const u = {
        resolution: gl.getUniformLocation(program, 'uResolution'),
        playheadSeconds: gl.getUniformLocation(program, 'uPlayheadSeconds'),
        timeSeconds: gl.getUniformLocation(program, 'uTimeSeconds'),
        visibleSeconds: gl.getUniformLocation(program, 'uVisibleSeconds'),
        durationSeconds: gl.getUniformLocation(program, 'uDurationSeconds'),
        colorAccent: gl.getUniformLocation(program, 'uColorAccent'),
        colorEdge: gl.getUniformLocation(program, 'uColorEdge'),
        hasDatum: gl.getUniformLocation(program, 'uHasDatum'),
        datum: gl.getUniformLocation(program, 'uDatum'),
        datumWidth: gl.getUniformLocation(program, 'uDatumWidth'),
        datumSampleCount: gl.getUniformLocation(program, 'uDatumSampleCount'),
        blobs: gl.getUniformLocation(program, 'uBlobs'),
        blobCount: gl.getUniformLocation(program, 'uBlobCount'),
    };
    for (const [name, loc] of Object.entries(u)) {
        if (loc === null) {
            console.warn(`${TAG} uniform '${name}' resolved to null — it will have no effect (misspelled or dead-stripped from the shader).`);
        }
    }

    // ── Mutable state, fed by the component through the handle. ──────────────────
    let datum: Datum | null = null;
    let playback: Playback = { positionSeconds: 0, isPlaying: false, pushWallClockMs: performance.now() };
    let visibleSeconds = DEFAULT_VISIBLE_SECONDS;

    // ── Lava physics control values (the R2 TEMP knob re-mapping — see the control-default
    // consts at the top of this file). These are the dials the existing knobs feed, routed
    // here by the handle setters. They drive the CPU physics step below, NOT a shader uniform.
    let lavaHeat = DEFAULT_DETACH;             // "Detach" knob  → heat
    let lavaGravity = DEFAULT_BUBBLYNESS;       // "Bubblyness" knob → gravity
    let collisionStrength = DEFAULT_COLOR_SHIFT_SPEED; // "Color-shift" knob → collision hardness
    let blobDensity = DEFAULT_BLOB_DENSITY;     // no live knob this wave (R4 adds it)

    /**
     * The *authoritative* playhead for this instant: the last pushed position advanced
     * by wall-clock elapsed since the push while playing, or the pushed position while
     * idle. The player remains the sole source of truth — this is display-only and is
     * never written back (read-only contract, spec §D / §5.10). This is the target the
     * rendered playhead converges onto; the shader uploads the *rendered* value (see
     * renderedPlayhead) so a re-anchor at a push doesn't snap on screen.
     */
    function effectivePlayhead(): number {
        if (!playback.isPlaying) return playback.positionSeconds;
        const elapsedSeconds = (performance.now() - playback.pushWallClockMs) / 1000;
        return playback.positionSeconds + elapsedSeconds;
    }

    // ── Rendered-playhead reconciliation (startup-jitter fix). ───────────────────────
    //
    // The shader scrolls to renderedPlayhead() = effectivePlayhead() + correctionOffset,
    // where correctionOffset decays exponentially toward 0 each frame (time constant
    // PLAYHEAD_CORRECTION_TIME_CONSTANT_SECONDS). At a push, setPlayback re-anchors the
    // authoritative target; without correction that re-anchor would teleport the
    // rendered playhead. Instead we *preserve the rendered position across the push* by
    // folding the discontinuity into correctionOffset (see setPlayback), then bleed it
    // off — turning each snap into a brief, sub-perceptible glide.
    //
    // Steady-state: when pushes are regular, the authoritative target barely moves at a
    // push, so the folded discontinuity is ~0 and correctionOffset stays ~0 — behaviour
    // is then identical to uploading effectivePlayhead() directly (the prior renderer).
    let correctionOffset = 0;
    let lastRenderWallClockMs = performance.now();

    /**
     * The playhead the shader actually scrolls to this frame. Equals the authoritative
     * effectivePlayhead() plus a correction offset that decays to zero, so the rendered
     * motion is continuous across the irregular startup pushes. Advances the decay by
     * real elapsed time since the previous render, making it frame-rate-independent
     * (same convergence on a 60 Hz and a 144 Hz display). Call exactly once per drawn
     * frame — it mutates the decay state.
     */
    function renderedPlayhead(): number {
        const nowMs = performance.now();
        const dtSeconds = Math.max(0, (nowMs - lastRenderWallClockMs) / 1000);
        lastRenderWallClockMs = nowMs;

        // Exponential decay of the error toward 0: offset *= e^(-dt/tau). Frame-rate
        // independent — the fraction retained depends only on wall-clock dt, not frame
        // count. Snap tiny residuals to 0 (an exponential never reaches it).
        if (correctionOffset !== 0) {
            correctionOffset *= Math.exp(-dtSeconds / PLAYHEAD_CORRECTION_TIME_CONSTANT_SECONDS);
            if (Math.abs(correctionOffset) < PLAYHEAD_CORRECTION_SNAP_SECONDS) correctionOffset = 0;
        }

        return effectivePlayhead() + correctionOffset;
    }

    /**
     * Resolve the navy↔moss field poles from the live palette vars on the canvas.
     *
     * Detects light vs dark by the page background luminance, then binds each pole to
     * the var that carries that identity in the active palette (see the note above the
     * ResolvedTheme interface for why no single var works across both modes):
     *   LIGHT: moss = --mud-palette-secondary (#3D7A68), navy = --mud-palette-primary (#17283f)
     *   DARK:  moss = --mud-palette-primary   (#3D7A68), navy = --mud-palette-background (#0D1B2A)
     * This yields the maximal navy↔moss spread the field wants in either theme.
     */
    function readTheme(): ResolvedTheme {
        const background = parseColor(readVar(canvas, '--mud-palette-background', '#FAFAF8'));
        const isDark = luminance(background) < 0.5;

        const moss = isDark
            ? parseColor(readVar(canvas, '--mud-palette-primary', '#3D7A68'))
            : parseColor(readVar(canvas, '--mud-palette-secondary', '#3D7A68'));
        const navy = isDark
            ? background // the dark ground (#0D1B2A) IS the navy pole on dark
            : parseColor(readVar(canvas, '--mud-palette-primary', '#17283f'));

        const resolved: ResolvedTheme = { accent: moss, edge: navy };
        // Report BOTH poles the R2 fill will use, as 0-255 RGB + relative luminance. (The
        // rich OKLab colour model is Wave R3; R2 just does a straight A→B theme fill — this
        // line confirms the navy/moss poles resolved off the canvas vars in the active mode.)
        const fmt = (c: [number, number, number]) =>
            `rgb(${Math.round(c[0] * 255)},${Math.round(c[1] * 255)},${Math.round(c[2] * 255)}) lum=${luminance(c).toFixed(2)}`;
        debugLog(`theme resolved (${isDark ? 'dark' : 'light'}) — MOSS(accent)=${fmt(moss)}  NAVY(edge)=${fmt(navy)}.`);
        return resolved;
    }

    let theme: ResolvedTheme = readTheme();

    // ════════════════════════════════════════════════════════════════════════════════
    //  R2 — the CPU wax-blob physics. Integrated each frame (real dt), then packed into
    //  `blobUpload` and sent to the shader as uBlobs[]. Allocation-free per frame: the
    //  blob pool and the upload buffer are built once here and mutated in place.
    //
    //  Space: height-normalized (pixel / canvasHeight). y ∈ [0,1] top→floor, x ∈ [0, aspect]
    //  where aspect = canvasWidth/canvasHeight. The FLOOR is y = 1 (the canvas bottom edge,
    //  already CSS-clipped to the footer top in R1). One isotropic unit → round blobs.
    // ════════════════════════════════════════════════════════════════════════════════

    interface Blob {
        x: number; y: number;   // centre, height-norm
        vx: number; vy: number; // velocity, height-norm/s
        r: number;              // radius, height-norm (fixed per blob, density-biased)
        temp: number;           // temperature 0..1
    }

    // The blob pool — MAX_BLOBS slots, all constructed once. liveCount (≤ MAX_BLOBS,
    // driven by the density dial) decides how many we simulate + upload this frame.
    const blobs: Blob[] = [];
    // The packed upload buffer (vec4 per blob). Reused every frame — no per-frame alloc.
    const blobUpload = new Float32Array(MAX_BLOBS * 4);

    /** Cheap deterministic PRNG (mulberry32) so blob spawn is varied but reproducible. */
    function makeRng(seed: number): () => number {
        let s = seed >>> 0;
        return () => {
            s = (s + 0x6d2b79f5) >>> 0;
            let t = s;
            t = Math.imul(t ^ (t >>> 15), t | 1);
            t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
            return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
        };
    }
    const rng = makeRng(0x1a2b3c4d);

    /** Construct (or re-seed) one blob at a random spot near the floor, ready to be heated. */
    function seedBlob(b: Blob, aspect: number): void {
        // Density biases radius toward the small end as it rises (more, smaller blobs).
        const radiusBias = 1 - blobDensity * 0.6; // density 0 → big, density 1 → smaller
        const r = (BLOB_RADIUS_MIN + rng() * (BLOB_RADIUS_MAX - BLOB_RADIUS_MIN)) * radiusBias;
        b.r = r;
        b.x = r + rng() * Math.max(aspect - 2 * r, 0.001); // somewhere across the width
        b.y = 1 - r - rng() * 0.1;                          // pooled near the floor
        b.vx = 0;
        b.vy = 0;
        b.temp = rng() * 0.3;                               // cool to start (heats at the floor)
    }

    /** (Re)build the whole pool — called once at setup and whenever the canvas aspect is first known. */
    function initBlobs(aspect: number): void {
        blobs.length = 0;
        for (let i = 0; i < MAX_BLOBS; i++) {
            const b: Blob = { x: 0, y: 0, vx: 0, vy: 0, r: 0, temp: 0 };
            seedBlob(b, aspect);
            blobs.push(b);
        }
    }
    let blobsInitialized = false;

    /** Live blob count for the current density dial, within [MIN_BLOB_COUNT, MAX_BLOBS]. */
    function liveBlobCount(): number {
        return Math.round(MIN_BLOB_COUNT + blobDensity * (MAX_BLOBS - MIN_BLOB_COUNT));
    }

    /**
     * CPU loudness sample at an absolute mix time, in [0,1], or 0 outside the mix. This
     * mirrors the shader's sampleAt() (same texel-centre convention) so the CPU collision
     * boundary matches the rendered waveform exactly. Reads the retained datum.samples.
     */
    function sampleLoudnessAt(timeSeconds: number): number {
        const d = datum;
        if (!d || timeSeconds < 0 || timeSeconds >= d.durationSeconds) return 0;
        const n = d.sampleCount;
        const p = (timeSeconds / d.durationSeconds) * n - 0.5;
        const i0 = Math.min(Math.max(Math.floor(p), 0), n - 1);
        const i1 = Math.min(Math.max(Math.floor(p) + 1, 0), n - 1);
        const f = Math.min(Math.max(p - Math.floor(p), 0), 1);
        const s0 = d.samples[i0] / 255;
        const s1 = d.samples[i1] / 255;
        return s0 + (s1 - s0) * f;
    }

    /** The heat dial's transfer function: dial 0..1 → how hard the floor pumps heat in.
     *  Designed so dial 0 = NO floor heating (wax rests, collision-only — §4c endpoint) and
     *  dial 1 = vigorous heating (many blobs go buoyant per second). A slight ease-in (square
     *  toe) keeps the low end gentle so small dial moves near 0 don't suddenly erupt. */
    function heatScaleFromDial(dial: number): number {
        const d = Math.min(Math.max(dial, 0), 1);
        return d * d * (3 - 2 * d); // smoothstep: flat at 0, steep in the middle, flat at 1
    }

    /** The collision-strength transfer: dial 0 = soft (penalty-spring, absorptive),
     *  dial 1 = hard (elastic, high restitution). Returns the restitution coefficient to
     *  use; the penalty-spring stiffness is held constant and the IMPULSE is scaled by the
     *  same dial so soft = mostly spring/no-bounce, hard = full elastic reflection (§5c). */
    function restitution(soft: number, hard: number): number {
        const d = Math.min(Math.max(collisionStrength, 0), 1);
        return soft + (hard - soft) * d;
    }

    /**
     * Advance the physics by dt seconds. Sub-stepped for spring stability. The collision
     * model: blob↔floor (soft contact), blob↔waveform (elastic deflect off the ribbon
     * surface normal, always on), blob↔blob (elastic, soft↔hard via the strength dial).
     */
    function stepPhysics(dtTotal: number): void {
        if (canvas.height <= 0) return;
        const aspect = canvas.width / canvas.height;

        if (!blobsInitialized) {
            initBlobs(aspect);
            blobsInitialized = true;
        }

        const count = liveBlobCount();
        const heatScale = heatScaleFromDial(lavaHeat);
        const gravity = GRAVITY_ACCEL_MIN + lavaGravity * (GRAVITY_ACCEL_MAX - GRAVITY_ACCEL_MIN);
        const collideRest = restitution(BLOB_RESTITUTION_SOFT, BLOB_RESTITUTION_HARD);
        const waveRest = restitution(WAVE_RESTITUTION_SOFT, WAVE_RESTITUTION_HARD);
        const collideHardness = Math.min(Math.max(collisionStrength, 0), 1);

        // Mix-time mapping at the current playhead (the waveform a blob's row sits over).
        const nowYn = NOW_ANCHOR_FROM_TOP;
        const secondsPerHeight = visibleSeconds;
        const centreX = aspect * 0.5;
        const maxHalf = (aspect * 0.5) * RIBBON_HALF_WIDTH_FRAC;
        const playhead = effectivePlayhead();

        const dt = Math.min(dtTotal, PHYSICS_MAX_DT) / PHYSICS_SUBSTEPS;

        for (let s = 0; s < PHYSICS_SUBSTEPS; s++) {
            // ── Per-blob: heat exchange, buoyancy, gravity, damping, floor contact. ──
            for (let i = 0; i < count; i++) {
                const b = blobs[i];

                // Heat exchange: floor heats (× heat dial), top cools, all relax to ambient.
                const distFromFloor = 1 - b.y;
                const distFromTop = b.y;
                if (distFromFloor < HEAT_FLOOR_ZONE) {
                    const near = 1 - distFromFloor / HEAT_FLOOR_ZONE; // 1 at floor → 0 at zone edge
                    b.temp += (1 - b.temp) * HEAT_FLOOR_RATE * heatScale * near * dt;
                }
                if (distFromTop < HEAT_TOP_ZONE) {
                    const near = 1 - distFromTop / HEAT_TOP_ZONE;
                    b.temp += (0 - b.temp) * HEAT_TOP_RATE * near * dt;
                }
                b.temp += (TEMP_AMBIENT - b.temp) * HEAT_AMBIENT_RATE * dt;
                b.temp = Math.min(Math.max(b.temp, 0), 1);

                // Forces: gravity down (+y), buoyancy from temperature (up = −y when hot).
                const buoyancy = BUOYANCY_COEFF * heatScale * (b.temp - TEMP_AMBIENT);
                b.vy += (gravity - buoyancy) * dt;

                // Viscous damping (lazy wax): frame-rate-independent exponential decay.
                const damp = Math.exp(-VISCOUS_DAMPING * dt);
                b.vx *= damp;
                b.vy *= damp;

                // Integrate position.
                b.x += b.vx * dt;
                b.y += b.vy * dt;

                // Floor: soft contact spring + extra damping so resting wax pools and flattens.
                const floorY = 1 - b.r;
                if (b.y > floorY) {
                    const pen = b.y - floorY;
                    b.vy -= FLOOR_SPRING * pen * dt;     // spring pushes up out of the floor
                    if (b.vy > 0) b.vy *= Math.exp(-FLOOR_CONTACT_DAMPING * dt); // kill the downward drive
                    b.y = floorY + pen * 0.5;            // ease the penetration out (soft, no snap)
                }
                // Ceiling: a gentle clamp so a very hot blob doesn't fly off-screen — it cools
                // at the top and falls back; just keep it inside the box.
                const ceilY = b.r;
                if (b.y < ceilY) { b.y = ceilY; if (b.vy < 0) b.vy = 0; }
                // Side walls: reflect softly so wax stays on screen.
                if (b.x < b.r) { b.x = b.r; if (b.vx < 0) b.vx = -b.vx * 0.3; }
                if (b.x > aspect - b.r) { b.x = aspect - b.r; if (b.vx > 0) b.vx = -b.vx * 0.3; }
            }

            // ── Blob ↔ waveform boundary (always on, independent of heat — §5b). ──
            // The waveform is a centred vertical band of half-width = loudness(row). A blob
            // whose centre is within (halfWidth + r) of the centre line penetrates it and is
            // pushed out along the band's surface normal (horizontal). Read-only authority:
            // the waveform is never moved, only the wax responds.
            for (let i = 0; i < count; i++) {
                const b = blobs[i];
                const t = playhead + (b.y - nowYn) * secondsPerHeight;
                const amp = sampleLoudnessAt(t);
                if (amp <= 0) continue;
                const halfW = amp * maxHalf;
                const dx = b.x - centreX;
                const sideSign = dx >= 0 ? 1 : -1;        // outward surface normal (in x)
                const penetration = halfW + b.r - Math.abs(dx);
                if (penetration > 0) {
                    // Soft penalty (the soft end of the dial): a spring proportional to the
                    // penetration depth pushes the wax out along the normal. Stronger as the
                    // dial → soft so the soft regime still recovers, just gently.
                    b.vx += sideSign * WAVE_COLLIDE_SPRING * penetration * dt * (1 - collideHardness * 0.5);

                    // Hard elastic (the hard end): reflect the velocity component going INTO
                    // the ribbon back out, scaled by restitution × hardness. inwardSpeed > 0
                    // means the blob is moving toward the centre line (into the surface).
                    const inwardSpeed = -sideSign * b.vx;
                    if (inwardSpeed > 0) {
                        // Remove the inward component and add back a restituted outward one.
                        b.vx += sideSign * inwardSpeed * (1 + waveRest) * collideHardness;
                    }

                    // Positional push-out: firm at the hard end (no penetration allowed),
                    // partial at the soft end (wax squishes in then eases out via the spring).
                    b.x += sideSign * penetration * (0.3 + 0.7 * collideHardness);
                }
            }

            // ── Blob ↔ blob (elastic 2D, soft↔hard via the strength dial — §5a). ──
            // O(count²) ≤ ~1k pair tests — trivial. Mass ∝ r² so big blobs shove small ones.
            for (let i = 0; i < count; i++) {
                const a = blobs[i];
                for (let j = i + 1; j < count; j++) {
                    const c = blobs[j];
                    let dx = c.x - a.x;
                    let dy = c.y - a.y;
                    let dist = Math.hypot(dx, dy);
                    const minDist = a.r + c.r;
                    if (dist >= minDist || dist <= 1e-6) continue;

                    const nx = dx / dist, ny = dy / dist; // collision normal a→c
                    const overlap = minDist - dist;
                    const ma = a.r * a.r, mc = c.r * c.r;  // mass ∝ area
                    const invSum = 1 / (ma + mc);

                    // Positional separation along the normal, mass-weighted (split the overlap).
                    const sep = overlap * (0.3 + 0.7 * collideHardness);
                    a.x -= nx * sep * (mc * invSum);
                    a.y -= ny * sep * (mc * invSum);
                    c.x += nx * sep * (ma * invSum);
                    c.y += ny * sep * (ma * invSum);

                    // Soft penalty spring along the normal (gentle shove, low strength).
                    const springAcc = BLOB_COLLIDE_SPRING * overlap * (1 - collideHardness * 0.6) * dt;
                    a.vx -= nx * springAcc; a.vy -= ny * springAcc;
                    c.vx += nx * springAcc; c.vy += ny * springAcc;

                    // Elastic impulse along the normal (hard end), with restitution + mass.
                    const rvx = c.vx - a.vx, rvy = c.vy - a.vy;
                    const velAlongNormal = rvx * nx + rvy * ny;
                    if (velAlongNormal < 0) { // approaching
                        const e = collideRest * collideHardness;
                        const impulse = -(1 + e) * velAlongNormal * invSum;
                        a.vx -= impulse * mc * nx; a.vy -= impulse * mc * ny;
                        c.vx += impulse * ma * nx; c.vy += impulse * ma * ny;
                    }
                }
            }
        }
    }

    /** Pack the live blobs into the upload buffer. Returns the live count. */
    function packBlobs(): number {
        const count = liveBlobCount();
        for (let i = 0; i < count; i++) {
            const b = blobs[i];
            const o = i * 4;
            blobUpload[o] = b.x;
            blobUpload[o + 1] = b.y;
            blobUpload[o + 2] = b.r;
            blobUpload[o + 3] = b.temp;
        }
        return count;
    }

    let rafId: number | null = null;
    let disposed = false;
    const startTimeMs = performance.now();
    // Wall-clock anchor for the physics dt (separate from the playhead decay clock).
    let lastPhysicsMs = performance.now();

    // FPS diagnostic (verification aid for the smoothness fix — gated on DEBUG). Counts
    // actual rAF callbacks and logs the rate ~once/sec while playing. This distinguishes
    // the two failure modes: a rate near the display refresh (~60) with the playhead
    // interpolated means motion is smooth; a rate near ~10 would mean the loop is gated
    // to the playback pushes instead of free-running. Reset when the loop (re)starts.
    let fpsFrameCount = 0;
    let fpsWindowStartMs = 0;

    // One-shot diagnostics: log the canvas dimensions + a post-draw gl.getError() the
    // first time we actually draw at a non-degenerate size. A 1×1 (or 300×150 default)
    // backing store here means the canvas had no layout box when the first draw ran —
    // the ResizeObserver will correct it, but the first paint would be degenerate.
    let firstRealDrawLogged = false;

    // Backing-store size in device pixels, tracked so we only resize the canvas
    // (which clears it) when the CSS box actually changed.
    let cssWidth = 0;
    let cssHeight = 0;
    let dpr = 1;

    // ── One-time GL pipeline setup. ──────────────────────────────────────────────
    gl.useProgram(program);
    gl.disable(gl.DEPTH_TEST);
    // Pre-multiplied alpha blend: src already carries colour*alpha (see frag shader).
    gl.enable(gl.BLEND);
    gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
    // The datum lives in texture unit 0 for the program's lifetime.
    gl.uniform1i(u.datum, 0);

    // ── ResizeObserver: one-shot redraw when the container changes while idle. ────
    //
    // The ResizeObserver is the SOLE size writer (spec §2e): we never call
    // getBoundingClientRect per frame. While playing, the rAF loop redraws every
    // frame anyway and picks up the new backing-store size set here. While idle, the
    // observer fires only on an actual size change and triggers a single redraw.
    const resizeObserver = new ResizeObserver((entries) => {
        if (disposed) return;
        const entry = entries[0];
        // contentBoxSize is the modern, layout-thrash-free size source. Fall back to
        // contentRect for engines that don't populate it.
        const box = entry.contentBoxSize?.[0];
        const nextCssWidth = box ? box.inlineSize : entry.contentRect.width;
        const nextCssHeight = box ? box.blockSize : entry.contentRect.height;
        applySize(nextCssWidth, nextCssHeight);
        // While idle, draw one still frame reflecting the new size. While playing,
        // the running loop will redraw on its next tick — no action needed.
        if (!playback.isPlaying) redrawOnce();
    });
    resizeObserver.observe(canvas);

    /**
     * Update the backing store to a CSS size × devicePixelRatio (capped at MAX_DPR)
     * and the GL viewport. Only resizes when something changed — resizing clears the
     * drawing buffer, so we avoid needless churn. This is the only place the canvas
     * size is written (fed by the ResizeObserver, never by a per-frame measure).
     */
    function applySize(nextCssWidth: number, nextCssHeight: number): void {
        const nextDpr = Math.min(window.devicePixelRatio || 1, MAX_DPR);
        if (nextCssWidth === cssWidth && nextCssHeight === cssHeight && nextDpr === dpr) {
            return;
        }
        cssWidth = nextCssWidth;
        cssHeight = nextCssHeight;
        dpr = nextDpr;
        canvas.width = Math.max(1, Math.round(cssWidth * dpr));
        canvas.height = Math.max(1, Math.round(cssHeight * dpr));
        gl.viewport(0, 0, canvas.width, canvas.height);
    }

    /**
     * Issue one GL draw with the current uniforms. The fragment shader does all the
     * scroll/zoom/ribbon work; here we just push the per-frame uniforms and draw the
     * full-screen triangle.
     */
    function draw(): void {
        if (canvas.width <= 0 || canvas.height <= 0) return;

        gl.clearColor(0, 0, 0, 0);
        gl.clear(gl.COLOR_BUFFER_BIT);

        gl.useProgram(program);
        gl.bindVertexArray(vao);

        // Per-frame uniforms. The playhead is the wall-clock-interpolated value, not
        // the raw last-pushed position — that is what makes the scroll advance every
        // animation frame instead of stepping at Blazor's ~10 Hz push cadence.
        gl.uniform2f(u.resolution, canvas.width, canvas.height);
        gl.uniform1f(u.playheadSeconds, renderedPlayhead());
        gl.uniform1f(u.timeSeconds, (performance.now() - startTimeMs) / 1000);
        // Per-change / per-theme / per-datum uniforms (cheap to set every frame; no
        // separate dirty-tracking needed for scalars/vec3s).
        gl.uniform1f(u.visibleSeconds, visibleSeconds);
        gl.uniform3fv(u.colorAccent, theme.accent);
        gl.uniform3fv(u.colorEdge, theme.edge);

        // Advance the wax-blob physics by the real elapsed time, then upload the blobs.
        // Stepping here (rather than in the loop) means idle one-shot redraws also advance
        // the sim by their actual dt — clamped by PHYSICS_MAX_DT, so a long paused gap just
        // means the lamp barely moves while paused (it animates with playback, spec §E).
        const nowMs = performance.now();
        const physicsDt = Math.max(0, (nowMs - lastPhysicsMs) / 1000);
        lastPhysicsMs = nowMs;
        stepPhysics(physicsDt);
        const liveCount = packBlobs();
        gl.uniform4fv(u.blobs, blobUpload);
        gl.uniform1i(u.blobCount, liveCount);

        if (datum) {
            gl.uniform1f(u.hasDatum, 1);
            gl.uniform1f(u.durationSeconds, datum.durationSeconds);
            gl.uniform1i(u.datumWidth, datum.texWidth);
            gl.uniform1i(u.datumSampleCount, datum.sampleCount);
            gl.activeTexture(gl.TEXTURE0);
            gl.bindTexture(gl.TEXTURE_2D, datum.texture);
        } else {
            gl.uniform1f(u.hasDatum, 0);
            gl.uniform1f(u.durationSeconds, 1);
            // Keep the divisor safe even though sampleAt early-outs on uHasDatum<0.5.
            gl.uniform1i(u.datumWidth, 1);
            gl.uniform1i(u.datumSampleCount, 1);
        }

        // One full-screen triangle (3 vertices), positions from gl_VertexID.
        gl.drawArrays(gl.TRIANGLES, 0, 3);
        gl.bindVertexArray(null);

        // First draw at a real (laid-out) size: report dimensions and any accumulated
        // GL error. We hold this log until cssWidth/cssHeight are populated so the
        // dimensions Daniel sees are the meaningful ones, not a pre-layout 1×1.
        // gl.getError() is a pipeline stall, so we only call it once, never per frame.
        if (!firstRealDrawLogged && cssWidth > 0 && cssHeight > 0) {
            firstRealDrawLogged = true;
            debugLog(
                `first draw — backing store ${canvas.width}x${canvas.height} px (css ${cssWidth}x${cssHeight} @ dpr ${dpr}), hasDatum=${datum ? 1 : 0}`,
            );
            const glErr = gl.getError();
            if (glErr !== gl.NO_ERROR) {
                console.error(`${TAG} gl.getError() after first draw: 0x${glErr.toString(16)} — the draw did not complete cleanly.`);
            }
        }
    }

    // ── rAF loop lifecycle (spec §E: cool when paused/backgrounded). ─────────────
    //
    // DESIGN: The loop runs ONLY while playing. When paused or stopped, no frames
    // are scheduled — the GPU is idle. The still slice stays correct via one-shot
    // redraws triggered by the handle methods (setZoom/refreshTheme/setDatum) and
    // by the ResizeObserver.
    //
    // Smoothness (spec §2e / §5.4): the scroll must advance every animation frame, not
    // step at Blazor's ~10 Hz playback-push cadence. We achieve that by interpolating
    // the playhead on the wall clock — each frame uploads renderedPlayhead() (= effectivePlayhead()
    // + the decaying jitter-correction offset), which advances the last pushed position by real time
    // elapsed since the push and blends out any accumulated timing error. (The separate uTimeSeconds
    // monotonic clock drives the blob-radius wobble in the shader; the CPU physics uses its own
    // wall-clock dt — neither drives the scroll, which is the playhead alone.)

    /** Draw one still frame immediately, without scheduling a new rAF. */
    function redrawOnce(): void {
        if (disposed) return;
        draw();
    }

    /** Start the rAF loop. No-op if already running or disposed (rafId guard). */
    function startLoop(): void {
        if (disposed || rafId !== null) return;
        // Reset the FPS window so the first measured second reflects the run we're
        // starting, not a stale count from a previous play session.
        fpsFrameCount = 0;
        fpsWindowStartMs = performance.now();
        // Re-base the decay clock to now so the first frame's dt is one frame, not the
        // (possibly long) idle gap since the last redrawOnce — otherwise a stale dt
        // would collapse the offset in one step. (Offset is 0 at play-start today, so
        // this is belt-and-braces, but it keeps the decay honest if that ever changes.)
        lastRenderWallClockMs = performance.now();
        // Re-base the physics clock too, so the first frame's dt is one frame, not the idle
        // gap since the last redraw (which would advance the lamp by a clamped jump on resume).
        lastPhysicsMs = performance.now();
        rafId = requestAnimationFrame(frame);
    }

    /** Stop the rAF loop. Safe to call when already stopped. */
    function stopLoop(): void {
        if (rafId !== null) {
            cancelAnimationFrame(rafId);
            rafId = null;
        }
    }

    /**
     * The animation loop. Runs only while playing. Each frame draws the scrolling
     * waveform at the wall-clock-interpolated playhead (effectivePlayhead, advancing
     * smoothly between the ~10 Hz pushes), then reschedules itself — unless playback
     * stopped since this frame was queued, in which case it draws one final still
     * frame (already done above) and exits the loop.
     *
     * A backgrounded tab gets rAF throttled by the browser automatically; on top of
     * that the loop does not run at all when paused, so a foregrounded-but-paused
     * mix burns no frames (spec §E / §5.3).
     */
    function frame(): void {
        if (disposed) {
            rafId = null;
            return;
        }
        draw();

        // FPS tally: count this callback, and once per elapsed second emit the rate.
        // performance.now() is cheap (no GPU stall, unlike gl.getError); the gated log
        // fires at most once/sec, so this adds no meaningful per-frame cost.
        if (DEBUG) {
            fpsFrameCount++;
            const nowMs = performance.now();
            const windowMs = nowMs - fpsWindowStartMs;
            if (windowMs >= 1000) {
                const fps = (fpsFrameCount * 1000) / windowMs;
                debugLog(`FPS ${fps.toFixed(1)} (${fpsFrameCount} frames in ${windowMs.toFixed(0)}ms) — playhead ${effectivePlayhead().toFixed(2)}s.`);
                // Lava diagnostic: the dials in play + how many blobs are currently buoyant
                // (temp above ambient) and how many are pooled on the floor. Daniel watches
                // this to confirm heat 0 = all-resting and heat-up = rising count climbs.
                const live = liveBlobCount();
                let buoyant = 0;
                let pooled = 0;
                let avgTemp = 0;
                for (let i = 0; i < live; i++) {
                    const b = blobs[i];
                    avgTemp += b.temp;
                    if (b.temp > TEMP_AMBIENT) buoyant++;
                    if (b.y > 1 - b.r - 0.04) pooled++;
                }
                debugLog(
                    `lava — heat=${lavaHeat.toFixed(2)} gravity=${lavaGravity.toFixed(2)} ` +
                    `collision=${collisionStrength.toFixed(2)} density=${blobDensity.toFixed(2)} | ` +
                    `blobs=${live} buoyant=${buoyant} pooled=${pooled} avgTemp=${(avgTemp / Math.max(live, 1)).toFixed(2)}.`,
                );
                fpsFrameCount = 0;
                fpsWindowStartMs = nowMs;
            }
        }

        if (playback.isPlaying) {
            rafId = requestAnimationFrame(frame);
        } else {
            // Playback stopped between queue and now; final still frame drawn above.
            rafId = null;
        }
    }

    // Read the initial size synchronously (one getBoundingClientRect at setup is
    // fine — it is the ResizeObserver that must not measure per-frame), then draw a
    // still frame so the canvas isn't blank before the first play command.
    {
        const rect = canvas.getBoundingClientRect();
        applySize(rect.width, rect.height);
        redrawOnce();
    }

    /**
     * Upload the loudness samples as a 2-D R8 texture that respects
     * GL_MAX_TEXTURE_SIZE, returning the Datum (with the grid dimensions the shader
     * needs to map a sample index → texel) or null on empty/invalid input.
     *
     * Why 2-D and not 1×N: the mix datum runs at ~333 samples/s, so any mix over
     * ~49 s produces more samples than GL_MAX_TEXTURE_SIZE (commonly 4096–16384),
     * and `texImage2D(…, width=N, height=1, …)` is rejected outright
     * ("Requested size at this level is unsupported"), leaving the waveform texture
     * uncreated and the ribbon blank. Laying the N samples row-major across a grid
     * of width = min(N, safeWidth) keeps every dimension well within the limit.
     *
     * Filtering: the shader reads with texelFetch and does its own time-axis
     * interpolation (see sampleAt), so NEAREST is correct here — hardware LINEAR on
     * a 2-D grid would bleed across the row-wrap seam. The final row is zero-padded
     * (texture init is zero-filled, then we overwrite the real samples); padding is
     * never read because sampleAt clamps the index to sampleCount-1.
     */
    function uploadDatum(samplesBase64: string, durationSeconds: number): Datum | null {
        if (durationSeconds <= 0 || !samplesBase64) {
            // Expected before the player reports a duration: the bridge pushes an empty
            // datum until then. Not an error, but worth seeing while diagnosing.
            debugLog(`uploadDatum skipped — durationSeconds=${durationSeconds}, base64 length=${samplesBase64?.length ?? 0}.`);
            return null;
        }
        const samples = decodeSamples(samplesBase64);
        const sampleCount = samples.length;
        if (sampleCount === 0) {
            console.warn(`${TAG} uploadDatum: decoded 0 samples from a non-empty base64 string — datum will not render.`);
            return null;
        }

        // Width = min(N, a safe power-of-two cap). The power-of-two cap (4096) is well
        // under every real GL_MAX_TEXTURE_SIZE and keeps row arithmetic clean; we
        // still clamp it to the actual max in case a driver reports something smaller.
        const SAFE_WIDTH = 4096;
        const texWidth = Math.min(sampleCount, Math.min(SAFE_WIDTH, maxTextureSize));
        const texHeight = Math.ceil(sampleCount / texWidth);
        debugLog(
            `uploadDatum — ${sampleCount} samples for ${durationSeconds.toFixed(2)}s mix ` +
            `(${(sampleCount / durationSeconds).toFixed(1)} samples/s); ` +
            `datum texture ${texWidth}x${texHeight} for N=${sampleCount} samples, maxTextureSize=${maxTextureSize}.`,
        );

        // Pad the final partial row with zeros so the full grid uploads in one call.
        const padded = texWidth * texHeight === sampleCount
            ? samples
            : (() => {
                const buf = new Uint8Array(texWidth * texHeight);
                buf.set(samples);
                return buf;
            })();

        const texture = gl.createTexture();
        if (!texture) return null;
        gl.activeTexture(gl.TEXTURE0);
        gl.bindTexture(gl.TEXTURE_2D, texture);
        // R8 rows are 1-byte-per-texel and texWidth is not guaranteed 4-aligned;
        // relax the default 4-byte unpack alignment so rows aren't read with stride
        // padding the source array doesn't have.
        gl.pixelStorei(gl.UNPACK_ALIGNMENT, 1);
        gl.texImage2D(
            gl.TEXTURE_2D, 0, gl.R8,
            texWidth, texHeight, 0,
            gl.RED, gl.UNSIGNED_BYTE, padded,
        );
        // NEAREST: texelFetch ignores the filter anyway, but be honest about it — the
        // shader interpolates manually to avoid the row-wrap seam (see sampleAt).
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
        gl.bindTexture(gl.TEXTURE_2D, null);

        return { texture, texWidth, texHeight, sampleCount, durationSeconds, samples };
    }

    return {
        setDatum(samplesBase64: string, durationSeconds: number): void {
            debugLog(`setDatum received — base64 length ${samplesBase64?.length ?? 0}, durationSeconds ${durationSeconds}.`);
            // Free the previous datum's GPU texture before replacing it (no leak
            // across re-pushes / mix changes — spec §5.11).
            if (datum) {
                gl.deleteTexture(datum.texture);
                datum = null;
            }
            datum = uploadDatum(samplesBase64, durationSeconds);
            // New datum changes what is drawn — refresh the still slice immediately
            // when idle. If playing, the running loop picks it up next frame.
            if (!playback.isPlaying) redrawOnce();
        },

        setPlayback(positionSeconds: number, isPlaying: boolean): void {
            const wasPlaying = playback.isPlaying;

            // Preserve on-screen continuity across the re-anchor. The rendered playhead
            // right now is effectivePlayhead() (old anchor) + correctionOffset; capture
            // it before we replace the anchor. We read effectivePlayhead() without going
            // through renderedPlayhead() so we don't advance the decay clock here — the
            // decay belongs to the render loop, ticked once per drawn frame.
            const renderedBefore = effectivePlayhead() + correctionOffset;

            // Anchor the pushed position to wall-clock NOW: the rAF loop interpolates
            // forward from here each frame (effectivePlayhead), so the scroll advances
            // smoothly between these ~10 Hz pushes.
            playback = { positionSeconds, isPlaying, pushWallClockMs: performance.now() };

            // Fold the re-anchor discontinuity into the correction offset so the rendered
            // playhead doesn't jump: choose offset such that effectivePlayhead() (new
            // anchor) + offset == renderedBefore. The render loop then decays this offset
            // to zero over PLAYHEAD_CORRECTION_TIME_CONSTANT_SECONDS, converging onto the
            // authoritative position. When pushes are regular the gap is ~0, so offset is
            // ~0 and steady-state matches the prior hard-anchor behaviour exactly.
            //
            // Only smooth while continuously playing. On a play/pause edge or while idle
            // we want the exact authoritative position, not a glide from a stale render:
            // a resume should land on the real position, and a paused still frame must be
            // truthful (read-only contract — never show a position the player isn't at).
            if (isPlaying && wasPlaying) {
                correctionOffset = renderedBefore - effectivePlayhead();
            } else {
                correctionOffset = 0;
            }

            if (isPlaying && !wasPlaying) {
                // Transition paused/stopped → playing: start the rAF loop.
                debugLog(`playback started — position ${positionSeconds.toFixed(2)}s, datum ${datum ? 'present' : 'ABSENT'}; starting rAF loop.`);
                startLoop();
            } else if (!isPlaying && wasPlaying) {
                // Transition playing → paused/stopped: the in-flight frame draws the
                // final still position and exits on its own (frame() checks
                // playback.isPlaying before rescheduling). We do NOT stopLoop() here —
                // that would cancel the in-flight frame before it draws, leaving a
                // stale canvas. Let the frame run out.
            }
            // isPlaying unchanged (position-only update): the running loop (if any)
            // redraws next frame; nothing to do here.
        },

        setZoom(seconds: number): void {
            // Clamp into the supported span so a stray value can't break the math.
            visibleSeconds = Math.min(MAX_VISIBLE_SECONDS, Math.max(MIN_VISIBLE_SECONDS, seconds));
            // While playing, the running rAF loop uploads uVisibleSeconds next frame; while idle the
            // loop is stopped (spec §E), so a zoom change must force one still frame here or the new
            // span is uploaded only on the next unrelated redraw (theme/datum/resize) — i.e. never.
            const idleRedraw = !playback.isPlaying;
            debugLog(`setZoom — requested ${seconds.toFixed(3)}s, clamped ${visibleSeconds.toFixed(3)}s; idleRedraw=${idleRedraw} (isPlaying=${playback.isPlaying}).`);
            if (idleRedraw) redrawOnce();
        },

        // ── R2 TEMPORARY control re-wiring (Wave R4 replaces this with the proper six-knob
        // set). The bridge still calls these three setters by their OLD names — the names are
        // a Wave-2 artifact and are NOT worth a bridge/contract change just to rename for one
        // wave. Each routes its [0,1] value to the lava-physics dial it now drives, so Daniel
        // can FEEL heat/gravity/collision in-browser this wave. The on-screen knob captions
        // still read the old labels (BubbleChart/Air/Palette) — R4 redraws the controls UI.
        //   setBubblyness     ← "Bubblyness" knob  → lava GRAVITY
        //   setDetach         ← "Detach" knob      → lava HEAT
        //   setColorShiftSpeed← "Color-shift" knob → COLLISION STRENGTH
        // Idle redraw mirrors setZoom so a paused tweak still updates the still frame.
        setBubblyness(value: number): void {
            lavaGravity = Math.min(1, Math.max(0, value)); // R2 TEMP → gravity
            debugLog(`setGravity (via setBubblyness) → ${lavaGravity.toFixed(3)}.`);
            if (!playback.isPlaying) redrawOnce();
        },

        setDetach(value: number): void {
            lavaHeat = Math.min(1, Math.max(0, value)); // R2 TEMP → heat
            debugLog(`setHeat (via setDetach) → ${lavaHeat.toFixed(3)}.`);
            if (!playback.isPlaying) redrawOnce();
        },

        setColorShiftSpeed(value: number): void {
            collisionStrength = Math.min(1, Math.max(0, value)); // R2 TEMP → collision hardness
            debugLog(`setCollisionStrength (via setColorShiftSpeed) → ${collisionStrength.toFixed(3)}.`);
            if (!playback.isPlaying) redrawOnce();
        },

        refreshTheme(): void {
            theme = readTheme();
            if (!playback.isPlaying) redrawOnce();
        },

        dispose(): void {
            disposed = true;
            stopLoop();
            resizeObserver.disconnect();
            // Release all GL resources so nothing leaks on navigation (spec §5.11).
            if (datum) {
                gl.deleteTexture(datum.texture);
                datum = null;
            }
            if (vao) gl.deleteVertexArray(vao);
            gl.deleteProgram(program);
        },
    };
}