# src/core/ui — pure UI geometry, palette, and interaction-decision modules

## Scope

Pure, REAPER-free UI geometry, palette, and interaction-decision modules shared by
the extension's docked bank panel and the VST3 instrument's editor/embed surfaces:
layout math, hit-testing, hover/drag-gesture-precedence decisions, and the
role-based color palette. Does **not** include: the actual LICE drawing (`draw_kit`
lives in `shell/panel`; the editor's own paint code lives in `shell/instrument`),
REAPER/SWELL window or dialog mechanics, or the DAW-side Design View flag
application (`shell/view`).

## Invariants

Look-and-feel — visual design language (Phase L) (settled decisions, 2026-07-26;
L7 sub-pass, 2026-07-27):

- **DS-1 — toolkit discipline.** Draw with LICE + reused WDL/vwnd pieces directly;
  external frameworks (iPlug2/JUCE/VSTGUI) are rejected. "**Caution, not a ban:**
  keep hit-test **geometry** in pure CTest-covered modules — do not import vwnd's
  retained-mode object model wholesale (its controls own their hit-test internally,
  which would move geometry into untestable shell code and undercut the pure/shell
  split)." This directory is that pure-geometry seam.
- **DS-2 — palette is role-based, not hardcoded hue**, in one constants block
  (`theme`): `bg/base`, `bg/panel`, `bg/cell`, `line/hairline`, `text/primary`,
  `text/dim`, `accent/primary`, `accent/secondary`, `accent/tertiary`,
  `accent/hot`, `overlay/trace`, `warn`. Neutral ladder sits in REAPER's mid-grey
  theme family (`bg/base` ≈ `#2b2b2b`, `bg/panel` ≈ `#333333`, `bg/cell` ≈
  `#3a3a3a`, `line/hairline` ≈ `#4a4a4a`, `text/primary` ≈ `#dcdcdc`, `text/dim` ≈
  `~#a0a0a0`+), elevation-ladder discipline (base < panel < cell by a few %,
  micro-gradient + inner highlight/shadow carry elevation, not hard borders). The
  accents carry categorical meaning: **primary (pastel lime) =
  live/active/selected**, secondary (deep teal) + tertiary (pastel purple) =
  supporting categorical distinctions (kinds, not intensity). `warn` (red/amber)
  is reserved **only** for byte-deleting or clip states (prune, delete).
  **The role count is not fixed at three** — a shade that works better in one spot
  earns its own named role (Daniel, 2026-07-31). What does not relax: draw sites
  ask by role. A literal color outside `theme`'s constants block is the violation,
  not the role count.
- **WCAG-floor discipline (tight pairs to re-verify on any palette change):**
  `text/dim` on `bg/panel`/`bg/cell` is the classic mid-grey-on-mid-grey floor
  failure — must clear AA 4.5:1 body text. The accents as state indicators/active
  fills on `bg/cell` have a shrunken contrast cushion (primary ~7.6:1, tertiary
  ~5.5:1, secondary only ~3.9:1 since it was darkened — all still clear 3:1, but
  re-check every one on any hue nudge). Body text on an accent fill is a tight AA
  4.5:1 pair (4.91:1 for `accent/secondary`). **The *binding* limiter on how dark
  `accent/secondary` may go is neither of those: it is the velocity-curve trace on
  a HOVER-lightened `bg/cell` at 3.03:1 against a 3:1 floor** — 1.2% headroom, the
  tightest pair in the tree, and invisible to any rest-surface check. Take the most
  pastel value that still clears the floor, approached from the soft side, never
  re-saturated toward neon "to be safe."
- **A state surface is a DIFFERENT surface.** Hover lightens toward `accent/hot`,
  pressed darkens, disabled desaturates — each changes the contrast of everything
  drawn on it. Enumerate the state surfaces a role is drawn over, not just the rest
  ones; the limiter above is exactly the pair that enumeration missed.
- **A mark drawn OVER an accent fill answers to two neighbours at once.** THE home
  for this rule. `overlay/trace` (the envelope trace on the waveform) must clear 3:1
  against both `accent/primary` and `bg/base`, which sit 9.41:1 apart — so
  `sqrt(9.41)` ≈ 3.07:1 is the ceiling for *any* single value, and the role sits at
  that optimum with no headroom either way. Three consequences, all load-bearing:
  the role is confined to the waveform band (no value clears 3:1 on `bg/cell` and
  against the lime at once); inside the 20%-alpha loop-span fill it is under floor
  at 2.25:1, and against `line/hairline`'s zero line at 1.92:1 — both KNOWN,
  ACCEPTED, and asserted as such, because no trace value fixes either; and any two
  values that BOTH clear the ceiling sit within 1.05:1 of each other, so a state of
  this mark (grabbed, hot, selected) **cannot** be carried by color and must be
  carried by size, fill-vs-outline, or shape.
- **"Speed is the selling point" — a design constraint on this geometry, not
  just the draw layer.** Sub-frame hover/press/drag feedback must repaint
  immediately on the input message. **No decorative animation** — no
  tweens/fades/pulses; the only permitted motion is a level/meter readout
  following audio directly. Any glow/bloom state is a static drawn state, never
  a pulse.
- **Precision/invariant implications (Phase L does not change these):** "the
  pure/shell split holds" — all layout/hit-test stays in pure CTest-covered
  geometry modules; the kit's *draw* half is shell, its *geometry* half is pure,
  even where a WDL piece is reused. Look-and-feel work never touches capture,
  placement, or bank data ownership.
- **Drag-gesture precedence is a pure decision helper, on both sides of the client
  rect.** Inside: drop on a tab/other bank → move/copy; else same-bank grid →
  reorder-to-slot (empty slot = place, occupied + no modifier =
  insert-before-and-shift, occupied + Alt = replace). Outside: `decideDropClass`
  resolves the surface under the cursor. The shell only reads live
  pointer/focus/client-rect/modifier state and calls these, then maps the resolved
  cue to a cursor via `SetCursor`. No cue or precedence logic belongs in the shell.
- **The drag-out law is per-move and stateless.** The class is resolved from the
  current pointer on every move and again at the release point; nothing is latched
  between evaluations. That is what makes every transition reversible and what
  makes drag speed (WM_MOUSEMOVE coalescing) unable to change an outcome. Leaving
  REAPER entirely is the one irreversible transition, because the OS hand-off goes
  modal. A first-move class lock and a drag-lifetime "cannot hand off" latch both
  existed here and were removed — do not reintroduce either.

## Modules

- `rect` (`core/ui`, header-only) — the ONE concrete pixel rectangle (Q-W1): XYWH storage + `right()`/`bottom()`/`ltrb()`/`contains()`, replacing 12+ byte-identical role structs (`ButtonRect`/`FooterRect`/`CellRect`/`KitBox`/…) and the VST side's separate LTRB `Rect`; every prior role name survives as a `using` alias at its old site (e.g. `editor_geometry::Rect`).
- `bank_grid` — REAPER-free grid layout, selection, keyboard-nav, and thumbnail-cache-key logic for the docked bank panel.
- `tab_strip` — REAPER-free scrollable tab-strip layout + hit-test for the named-banks strip.
- `prune_button` — pure layout/hit-test for the `bank_panel` footer Prune button.
- `drag_out` — the pure drag-out gesture law plus path-list assembly. Owns the `ReaperSurface` vocabulary (OffReaper / TrackPanel / FxSurface / FxEmbed / Arrange / Other — `core/wire/instrument_drop` classifies REAPER's info token INTO it), `decideDropClass` (surface × single-vs-multi payload → Internal / InstrumentDrop / ArrangeInsert / Refuse / OsHandoff / None), and `cueForDropClass`. **No `DropClass` means "nothing happens"**: a surface with no defined outcome for the payload resolves to `Refuse`, which the shell shows as a cursor, so "no silent no-op release" is a property of the enumeration rather than of any call site.
- `theme` — pure palette module: role→color mapping, REAPER-grey neutral ladder + the pastel accent system, the keyboard strip's spectral ramp, WCAG contrast-floor helpers + `compositeOver` (the effective color of a translucent fill, so alpha overlays are testable). Only the ramp's MID stop is its own constant; lo/hi are still aliases of `accent/primary`/`accent/tertiary`, so a categorical accent move CAN still reorder the ramp — `testSpectralRampLuminanceIsMonotonic` is the build-time catch, not the structure.
- `component_geometry` — pure button/slider/list-row geometry + hover hit-test helpers.
- `action_bar` — pure task-grouped action-bar layout/hit-test: clusters (Capture / Placement / Maintenance / Tagging / Switching).
- `footer_bar` — pure footer layout/hit-test: `[Arrange|Design]` mode-toggle geometry, Tail button, and Prune placement, plus `modeSegmentEnabled` — the mode segment's live/dead predicate under the playback gate AND under whether the shell resolved a routable command id for it (both bools passed IN, so this stays REAPER-free).
- `overflow_menu` — pure overflow-menu-button geometry/reserve/hit-test for the top-toolbar More (⋯) button.
- `mode_enable` — pure opposite-mode enablement predicate: given the active mode, computes per-button live/disabled state for the four Item/Track × Arrange/Design tag buttons.
- `tooltip` — pure tooltip placement + prefix-strip: strips the `ReaSampler:` display prefix from the registered action phrase; width clamped to the client rect.
- `card_drag` — pure drag-gesture precedence + slot hit-test: leave-client → OS drag-out; other-bank → move/copy; same-bank → reorder / Alt-over-occupied → replace.
- `card_meta` — pure card-metadata formatters: bars.beats.subdivisions and seconds.milliseconds; blank when the sample is unstamped. Also `cardNameStrip` + the two strip constants — the card's name line sits across the TOP of the cell, drawn over the waveform exactly as the length read-out is over it at the bottom, and is suppressed entirely on a cell with no room for both strips plus a waveform band.
- `stroke_aa` — analytic antialiased thick-stroke COVERAGE (the shell blends it): `StrokeCanvas`, a reusable mask holding distance-to-polyline coverage MAX-accumulated across segments, plus `strokePolyline` / `strokeBounds` / `appendArc` / `rasterRowOffset` (the row-major offset
  math for a possibly bottom-up raster, pulled out of the shell's LICE blend so its flipped
  branch is pinned by a host-free test). An arc is just a flattened polyline, so ONE path serves the knob arcs, the inner dial, the envelope polyline and both spline traces. Coverage is `clamp(halfWidth + 0.5 - distance, 0, 1)`, which makes perpendicular weight exactly `2·halfWidth` at every angle. **The guaranteed-opaque-core threshold is width ≥ 2 px, not any width above 1 px**: opacity needs `distance <= halfWidth - 0.5`, and the worst-case distance from a pixel centre to the centreline is 0.5, so a 1 px stroke (`halfWidth = 0.5`) has zero slack — its peak alpha modulates with the stroke's exact alignment to the pixel grid instead of pinning to 255 (Daniel's ruling, 2026-08-01: every stroker-drawn width on the editor is now >= 2 px for this reason — `testSubOpaqueCoreAtOnePixelWidth` in `tests/test_stroke_aa.cpp` still pins the 1 px case as a property of the stroker, independent of whether any surface ships at that width). Long segments are subdivided before rasterizing — EXACT, not an approximation (min-distance to a partition of a segment is min-distance to the whole), purely to keep each piece's bounding box tight, since one long diagonal's box has area O(len²).

## Gotchas

- The WCAG contrast-floor pairs above are real math in `theme`'s tests, not a
  visual eyeball check — any new hue or role needs its own contrast-floor
  assertion. Enumerate the draw sites by grep, not from memory: the enumeration
  must cover state surfaces (hover/pressed/active), translucent composites, and
  anything else drawn into the same rect, not just the rest surfaces.
- **A text color's floor is set by its FONT, not by its role.** WCAG's large-scale
  class needs ≥24px, or ≥18.66px BOLD (`theme.h`'s thresholds); anything smaller is
  body class and answers to 4.5:1. A draw site relying on the 3:1 floor must pin its
  font metrics against those constants — `draw_kit.cpp`'s `Font::RegionTitle` is the
  worked example. A prior revision wrote the threshold ~25% low and let 15px
  semibold clear a floor it was not entitled to.
- `card_drag`'s precedence order must stay a pure decision helper mirroring
  `drag_out::decideDropClass` — don't let a shell reimplement gesture precedence
  ad hoc; the cursor-cue mapping in the shell must stay a thin lookup over the
  pure result.
- `drag_out` is deliberately **dependency-free**, and `instrument_drop` links it
  rather than the reverse. Inverting that edge would drag the instrument's state
  serializer into `card_drag` and into every `drag_out` consumer.
- `rect`'s prior role names survive only as `using` aliases at their old call
  sites — changing `rect.h` itself ripples across every directory that aliases
  it (e.g. `editor_geometry::Rect`); check all alias sites, not just this one.
- **`stroke_aa`'s mask is deliberately NOT cleared on `reset`.** Only
  `[rowLo, rowHi)` of each row holds meaningful coverage; everything else is
  whatever the reused buffer last held. That is what keeps a stroke's cost
  proportional to its ink rather than to its bounding box — but it means any new
  reader must respect the row extents, and any new writer must grow them through
  `extendRow`, which zero-fills the newly-valid cells INCLUDING the gap when a
  stroke revisits a row far from where it left it (a circle touches most rows on
  both sides). Reading the raw buffer outside the extents returns garbage by
  design, not zero.
- **Neither `LICE_Arc` nor `LICE_ThickFLine` can draw these strokes** — the first
  never reaches an opaque core, the second's width is along the minor axis so its
  perpendicular weight falls off as `cos θ`. The evidence and the measurements
  live in `docs/product/visual-design-language.md` §8; do not "simplify" a stroke
  site back onto either primitive.
