Merge Phase 21.3 (seek-back-past-window refill + AC6 recovery) into streaming-overhaul

DeepDrftPublic.Client/Services/AudioInteropService.cs

@@ -183,6 +183,18 @@ public class AudioInteropService : IAsyncDisposable
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.reinitializeFromOffset", playerId, totalStreamLength, seekPosition);
     }
 
+    /// <summary>
+    /// Phase 21.3 / AC6 clean-failure recovery: after a window-miss refill (seek-back past the
+    /// retained tail) fails its Range fetch or reinit, halt the starved scheduler and leave the
+    /// player paused-but-loaded at <paramref name="seekPosition"/> so no silent false end fires and a
+    /// retry is possible. Routes through <see cref="InvokeJsAsync{T}"/> so an interop failure during
+    /// recovery still yields a failure result rather than throwing into the seek path.
+    /// </summary>
+    public async Task<AudioOperationResult> RecoverFromFailedRefill(string playerId, double seekPosition)
+    {
+        return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.recoverFromFailedRefill", playerId, seekPosition);
+    }
+
     public async Task<AudioOperationResult> SetVolumeAsync(string playerId, double volume)
     {
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.setVolume", playerId, volume);

DeepDrftPublic.Client/Services/AudioPlayerService.cs

@@ -118,6 +118,16 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
                     IsPlaying = true;
                     IsPaused = false;
                 }
+                else if (IsPaused)
+                {
+                    // Play failed while the player is paused — the scheduler may be empty after a
+                    // failed refill (AC6 recovery). Re-issue a seek at the current position: the
+                    // seek path routes to seekBeyondBuffer when the scheduler is empty (Phase 21.3
+                    // fix), triggering a real refetch rather than returning "Streaming not ready".
+                    // We return early here; Seek owns its own state mutations and NotifyStateChanged.
+                    await Seek(CurrentTime);
+                    return;
+                }
             }
 
             if (!result.Success)

DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs

@@ -629,6 +629,15 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
         await DrainActiveStreamingTaskAsync();
         oldCts?.Dispose();
 
+        // Single-writer discipline (C6/AC8): all three failure exits must share the same guard.
+        // TrackMediaClient.GetTrackMedia swallows OperationCanceledException and returns
+        // Success==false, so a superseded seek lands in the media-fetch-fail branch below
+        // rather than in the OCE catch. Without the guard those branches would call
+        // RecoverFromFailedRefill — running clearForSeek + setPlaybackOffset against the player
+        // state the NEWER seek now owns. A local predicate keeps all three exits symmetric so a
+        // future exit cannot forget the check.
+        bool IsStillActiveSeek() => ReferenceEquals(_streamingCancellation, seekCts);
+
         try
         {
             // Update UI immediately
@@ -649,8 +658,15 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
             {
                 var technicalError = mediaResult.GetMessage() ?? "Failed to load audio from position";
                 _logger.LogError("Failed to get track media from offset {Offset}: {Error}", byteOffset, technicalError);
-                ErrorMessage = StreamingErrorHandler.GetUserFriendlyMessage(technicalError);
-                IsSeekingBeyondBuffer = false;
+                // Guard: a superseded seek must NOT touch shared state. The newer seek owns teardown.
+                if (IsStillActiveSeek())
+                {
+                    await RecoverFromFailedRefill(seekPosition, StreamingErrorHandler.GetUserFriendlyMessage(technicalError));
+                }
+                else
+                {
+                    _logger.LogDebug("Media-fetch failed on superseded seek to {Position} — newer seek owns state, skipping recovery", seekPosition);
+                }
                 return;
             }
 
@@ -661,8 +677,15 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
             if (!reinitResult.Success)
             {
                 _logger.LogError("Failed to reinitialize for offset streaming: {Error}", reinitResult.Error);
-                ErrorMessage = "Failed to seek to position";
-                IsSeekingBeyondBuffer = false;
+                // Guard: same single-writer discipline — only recover when we are still the active seek.
+                if (IsStillActiveSeek())
+                {
+                    await RecoverFromFailedRefill(seekPosition, "Failed to seek to position");
+                }
+                else
+                {
+                    _logger.LogDebug("Reinit failed on superseded seek to {Position} — newer seek owns state, skipping recovery", seekPosition);
+                }
                 return;
             }
 
@@ -684,19 +707,55 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
             // still the active seek — if _streamingCancellation has been replaced, a
             // newer seek is in progress and owns the flag.
             _logger.LogDebug("Seek beyond buffer cancelled");
-            if (ReferenceEquals(_streamingCancellation, seekCts))
+            if (IsStillActiveSeek())
             {
                 IsSeekingBeyondBuffer = false;
             }
         }
         catch (Exception ex)
         {
+            // A refill fetch can fail deep into a long mix (the listener didn't initiate it). Recover
+            // into a clean paused-but-loaded state (AC6) rather than leaving the starved scheduler to
+            // fire a silent false end. Only when we are still the active seek — a superseding seek owns
+            // the state and the OCE catch above handles its own teardown.
             _logger.LogError(ex, "Error during seek beyond buffer to position {Position}", seekPosition);
-            ErrorMessage = StreamingErrorHandler.GetUserFriendlyMessage(ex.Message);
+            if (IsStillActiveSeek())
+            {
+                await RecoverFromFailedRefill(seekPosition, StreamingErrorHandler.GetUserFriendlyMessage(ex.Message));
+            }
+        }
+    }
+
+    /// <summary>
+    /// Clean-failure recovery for a window-miss refill (Phase 21.3 / AC6). A backward seek past the
+    /// retained tail re-fetches via the existing Range path; that mid-stream fetch the listener did not
+    /// initiate can fail deep into a long mix. When it does, the pre-seek loop has already been
+    /// cancelled and drained, but the JS scheduler is still holding stale pre-seek buffers and still
+    /// "playing" — left alone it drains them and fires a silent false end (the wedged/starved state AC6
+    /// forbids). This halts the scheduler into a paused-but-loaded state at <paramref name="seekPosition"/>,
+    /// surfaces a clear error, and leaves the track loaded so the listener can retry the seek or pick
+    /// another track. Mirrors <c>PlaybackScheduler.playFromPosition</c>'s end-of-buffer recovery: stop
+    /// pretending to play.
+    /// </summary>
+    private async Task RecoverFromFailedRefill(double seekPosition, string userFacingError)
+    {
+        // Halt the starved scheduler JS-side (stop sources, drop stale buffers, anchor at the target).
+        // Best-effort: if even this interop fails the player is no worse off, and we still surface the
+        // error and settle C# state below.
+        var recovered = await _audioInterop.RecoverFromFailedRefill(PlayerId, seekPosition);
+        if (!recovered.Success)
+        {
+            _logger.LogWarning("Refill-failure recovery interop did not succeed: {Error}", recovered.Error);
+        }
+
+        // Settle C# into the matching recoverable state: not playing, paused at the target, still loaded.
+        ErrorMessage = userFacingError;
+        IsPlaying = false;
+        IsPaused = true;
+        CurrentTime = seekPosition;
         IsSeekingBeyondBuffer = false;
         await NotifyStateChanged();
     }
-    }
 
     /// <summary>
     /// Single method to reset all state - called by both Stop and Unload.

DeepDrftPublic/Interop/audio/AudioPlayer.test.ts

@@ -0,0 +1,298 @@
+/**
+ * AudioPlayer window-miss refill tests (Phase 21.3) — the seek-dispatch TRIGGER and the AC6
+ * clean-failure recovery.
+ *
+ * What this pins (the genuinely-new 21.3 work):
+ *   - The window-miss TRIGGER. AudioPlayer.seek() routes by whether the target falls inside the
+ *     retained window [playbackOffset, playbackOffset + totalDuration]. After 21.1 partial eviction
+ *     playbackOffset is the absolute start of the retained back-window tail, so:
+ *       * seek back WITHIN the tail  -> seekWithinBuffer, NO refetch (UC3 / AC4),
+ *       * seek back PAST the tail     -> seekBeyondBuffer with the EARLIER resolved offset (UC4 / AC5),
+ *         using whichever resolver the active path ships (WAV calculateByteOffset; Opus
+ *         resolveOpusByteOffset over the sidecar index),
+ *       * seek forward past the decoded end -> seekBeyondBuffer forward, unchanged (UC2/UC5).
+ *   - The AC6 recovery. recoverFromFailedRefill() halts the scheduler (clearForSeek), anchors the
+ *     offset at the seek target, and leaves the player paused-but-loaded so no silent false end fires.
+ *
+ * The seek dispatch and recovery are pure given the scheduler + active decoder, so they are testable
+ * in Node by white-box-injecting fakes for `scheduler`, `streamDecoder`, and `opusDecoder` (the same
+ * private-field injection idiom the scheduler/Opus tests use). The AudioPlayer constructor itself is
+ * Node-safe: it builds AudioContextManager/StreamDecoder/PlaybackScheduler, none of which touch Web
+ * Audio until initialize(). No AudioContext, no WebCodecs.
+ *
+ * Same harness convention as the sibling tests (no runner in this repo); run a copy from the COMPILED
+ * output so the `.js` import specifiers resolve:
+ *
+ *   dotnet build DeepDrftPublic/DeepDrftPublic.csproj
+ *   cp DeepDrftPublic/Interop/audio/AudioPlayer.test.ts DeepDrftPublic/wwwroot/js/audio/
+ *   node DeepDrftPublic/wwwroot/js/audio/AudioPlayer.test.ts
+ *
+ * A thrown error / non-zero exit signals failure; "ALL <n> TESTS PASSED" signals success.
+ * Excluded from the production tsc build via tsconfig `exclude: Interop/ ** /*.test.ts`.
+ */
+
+import { AudioPlayer } from './AudioPlayer.js';
+import { parseSidecar } from './OpusSidecar.js';
+import type { OpusSeekData } from './OpusSidecar.js';
+
+// --- tiny inline harness (no dependencies) ---------------------------------------------------
+let passed = 0;
+const failures: string[] = [];
+function test(name: string, fn: () => void): void {
+    try {
+        fn();
+        passed++;
+    } catch (e) {
+        failures.push(`FAIL: ${name}\n    ${(e as Error).message}`);
+    }
+}
+function assertEqual(actual: unknown, expected: unknown, msg?: string): void {
+    if (actual !== expected) {
+        throw new Error(`${msg ?? 'assertEqual'}: expected ${String(expected)}, got ${String(actual)}`);
+    }
+}
+function assertTrue(cond: boolean, msg?: string): void {
+    if (!cond) throw new Error(msg ?? 'assertTrue failed');
+}
+
+// --- fakes -----------------------------------------------------------------------------------
+
+/**
+ * A scheduler stand-in exposing only what AudioPlayer.seek / seekWithinBuffer / seekBeyondBuffer /
+ * recoverFromFailedRefill read or call. The retained window is [offset, offset + total]. Records the
+ * methods that mutate so the recovery test can assert the cleanup happened.
+ */
+class FakeScheduler {
+    private offset: number;
+    private total: number;
+    // hasBuffers reflects whether the scheduler holds decoded audio. Starts true when total > 0
+    // (a populated window), set to false by clearForSeek() (recovery drains the buffers).
+    private _hasBuffers: boolean;
+    public clearedForSeek = false;
+    public stoppedAllSources = false;
+    public offsetSetTo: number | null = null;
+    constructor(offset: number, total: number) {
+        this.offset = offset;
+        this.total = total;
+        this._hasBuffers = total > 0;
+    }
+
+    getPlaybackOffset(): number { return this.offset; }
+    getTotalDuration(): number { return this.total; }
+    hasBuffers(): boolean { return this._hasBuffers; }
+    stopAllSources(): void { this.stoppedAllSources = true; }
+    // seekWithinBuffer calls playFromPosition only when wasPlaying; isPlaying is false in these
+    // unit constructions, so it is never invoked — present for completeness.
+    playFromPosition(_position: number): void { /* no-op */ }
+    clearForSeek(): void { this.clearedForSeek = true; this._hasBuffers = false; }
+    setPlaybackOffset(o: number): void { this.offset = o; this.offsetSetTo = o; }
+}
+
+/** A StreamDecoder stand-in for the WAV path: a format is parsed and byte math is identity-scaled. */
+class FakeStreamDecoder {
+    private hasFormat: boolean;
+    private bytesPerSecond: number;
+    public requestedOffsetFor: number | null = null;
+    constructor(hasFormat: boolean, bytesPerSecond: number) { this.hasFormat = hasFormat; this.bytesPerSecond = bytesPerSecond; }
+    getFormatInfo(): unknown { return this.hasFormat ? { ok: true } : null; }
+    calculateByteOffset(position: number): number {
+        this.requestedOffsetFor = position;
+        return Math.round(position * this.bytesPerSecond);
+    }
+}
+
+function makePlayer(): AudioPlayer {
+    // Constructor is Node-safe (no Web Audio until initialize()).
+    return new AudioPlayer();
+}
+
+/** Inject the seek-relevant private fields and put the player in a loaded/streaming/playing state. */
+function arm(
+    player: AudioPlayer,
+    opts: {
+        scheduler: FakeScheduler;
+        duration: number;
+        streamDecoder?: FakeStreamDecoder;
+        opusDecoder?: object | null;
+        sidecar?: OpusSeekData | null;
+    }
+): void {
+    const priv = player as unknown as Record<string, unknown>;
+    priv.scheduler = opts.scheduler;
+    priv.duration = opts.duration;
+    priv.isStreamingMode = true;
+    priv.isPlaying = false; // keep dispatch pure (no real playFromPosition needed)
+    if (opts.streamDecoder) priv.streamDecoder = opts.streamDecoder;
+    priv.opusDecoder = opts.opusDecoder ?? null;
+    priv.activeOpusSidecar = opts.sidecar ?? null;
+}
+
+/** Read back private fields the recovery sets. */
+function priv(player: AudioPlayer): Record<string, unknown> {
+    return player as unknown as Record<string, unknown>;
+}
+
+// A minimal real sidecar (parsed) so the Opus resolver returns a deterministic page offset.
+// Index: t=0 -> byte 4096, t=1s -> byte 9000 (granule uses 48 kHz + preSkip).
+function makeOpusSidecar(): OpusSeekData {
+    const setup = [0x4f, 0x70, 0x75, 0x73, 0x48, 0x65, 0x61, 0x64];
+    const SEEK_INDEX_HEADER_SIZE = 24;
+    const SEEK_POINT_SIZE = 16;
+    const preSkip = 312;
+    const points = [
+        { granule: preSkip, byteOffset: 4096 },              // t = 0
+        { granule: preSkip + 48000, byteOffset: 9000 },      // t = 1 s
+    ];
+    const total = 4 + setup.length + SEEK_INDEX_HEADER_SIZE + points.length * SEEK_POINT_SIZE;
+    const bytes = new Uint8Array(total);
+    const view = new DataView(bytes.buffer);
+    view.setUint32(0, setup.length, true);
+    bytes.set(setup, 4);
+    let p = 4 + setup.length;
+    const writeU64 = (off: number, v: number) => {
+        view.setUint32(off, v >>> 0, true);
+        view.setUint32(off + 4, Math.floor(v / 0x100000000), true);
+    };
+    writeU64(p, 500_000);
+    view.setFloat64(p + 8, 100, true);
+    view.setUint32(p + 16, points.length, true);
+    view.setUint16(p + 20, preSkip, true);
+    p += SEEK_INDEX_HEADER_SIZE;
+    for (const pt of points) { writeU64(p, pt.granule); writeU64(p + 8, pt.byteOffset); p += SEEK_POINT_SIZE; }
+    const parsed = parseSidecar(bytes);
+    if (!parsed) throw new Error('test setup: sidecar failed to parse');
+    return parsed;
+}
+
+// --- TRIGGER: within-window vs past-tail vs forward ------------------------------------------
+
+// UC3 / AC4: a backward seek INTO the retained tail resolves from buffer — NO seekBeyondBuffer,
+// NO refetch signal. Window is [30, 60); target 40 is inside.
+test('seek back within retained tail resolves in-buffer (no refetch) — AC4', () => {
+    const player = makePlayer();
+    const scheduler = new FakeScheduler(30, 30); // retained window [30, 60)
+    arm(player, { scheduler, duration: 120, streamDecoder: new FakeStreamDecoder(true, 1000) });
+
+    const result = player.seek(40);
+    assertEqual(result.success, true, 'seek succeeds');
+    assertEqual(result.seekBeyondBuffer ?? false, false, 'within-window seek does NOT signal a refetch');
+    // No clearForSeek / no offset request — the retained window served it.
+    assertEqual(scheduler.clearedForSeek, false, 'no clearForSeek for an in-buffer seek');
+});
+
+// UC4 / AC5 (WAV): a backward seek PAST the retained tail signals a refill at the EARLIER resolved
+// offset, using the WAV resolver. Window [30, 60); target 10 is before the tail.
+test('seek back past retained tail refetches at the WAV-resolved earlier offset — AC5', () => {
+    const player = makePlayer();
+    const scheduler = new FakeScheduler(30, 30); // retained window [30, 60)
+    const wav = new FakeStreamDecoder(true, 2000); // 2000 bytes/sec
+    arm(player, { scheduler, duration: 120, streamDecoder: wav });
+
+    const result = player.seek(10); // earlier than the retained tail start (30)
+    assertEqual(result.success, true, 'seek succeeds');
+    assertEqual(result.seekBeyondBuffer, true, 'past-tail back seek signals a refill (window miss)');
+    assertEqual(wav.requestedOffsetFor, 10, 'WAV resolver consulted for the EARLIER target');
+    assertEqual(result.byteOffset, 20000, 'refill offset is the WAV-resolved earlier byte offset');
+});
+
+// UC4 / AC5 (Opus): the same window miss on the Opus path uses resolveOpusByteOffset over the
+// sidecar index (the live seek), not WAV byte math. Target 0.3 s resolves to the t=0 page (4096).
+test('seek back past retained tail refetches at the Opus index-resolved offset — AC5', () => {
+    const player = makePlayer();
+    const scheduler = new FakeScheduler(30, 30); // retained window [30, 60)
+    arm(player, {
+        scheduler,
+        duration: 100,
+        opusDecoder: {}, // presence routes seekBeyondBuffer down the Opus branch
+        sidecar: makeOpusSidecar(),
+    });
+
+    const result = player.seek(0.3); // earlier than the retained tail (30) -> window miss
+    assertEqual(result.success, true, 'seek succeeds');
+    assertEqual(result.seekBeyondBuffer, true, 'past-tail back seek signals a refill on Opus too');
+    assertEqual(result.byteOffset, 4096, 'Opus index resolved the t=0 page start for the earlier target');
+    // The landing time of the resolved page is captured for the decoder lead-trim (AC9 reuse).
+    assertEqual(priv(player)._seekLandingTime, 0, 'landing time of the resolved page captured for lead-trim');
+});
+
+// UC2/UC5: a forward seek past the decoded end still routes to seekBeyondBuffer forward, unchanged.
+test('forward seek past decoded end still routes to seekBeyondBuffer (unchanged)', () => {
+    const player = makePlayer();
+    const scheduler = new FakeScheduler(30, 30); // decoded [30, 60)
+    const wav = new FakeStreamDecoder(true, 1500);
+    arm(player, { scheduler, duration: 120, streamDecoder: wav });
+
+    const result = player.seek(90); // past the decoded end (60)
+    assertEqual(result.seekBeyondBuffer, true, 'forward-beyond-buffer still signals a fetch');
+    assertEqual(wav.requestedOffsetFor, 90, 'forward target resolved through the same WAV resolver');
+    assertEqual(result.byteOffset, 135000, 'forward offset is the resolved later byte offset');
+});
+
+// --- AC6: clean-failure recovery -------------------------------------------------------------
+
+// A failed refill must leave the player recoverable: scheduler halted (clearForSeek), offset anchored
+// at the seek target, paused-but-loaded — never a starved "playing" scheduler that fires a false end.
+test('recoverFromFailedRefill halts the scheduler and leaves a paused-but-loaded state — AC6', () => {
+    const player = makePlayer();
+    const scheduler = new FakeScheduler(30, 30);
+    arm(player, { scheduler, duration: 120, streamDecoder: new FakeStreamDecoder(true, 1000) });
+    // Simulate the pre-failure "playing" state the drained pre-seek loop leaves behind.
+    priv(player).isPlaying = true;
+    priv(player).isPaused = false;
+    priv(player).streamingStarted = true;
+
+    const result = player.recoverFromFailedRefill(15);
+    assertEqual(result.success, true, 'recovery succeeds');
+    assertTrue(scheduler.clearedForSeek, 'stale buffers dropped (no false end can fire)');
+    assertEqual(scheduler.offsetSetTo, 15, 'offset anchored at the seek target for a retry');
+    assertEqual(priv(player).isPlaying, false, 'not playing after recovery');
+    assertEqual(priv(player).isPaused, true, 'paused after recovery');
+    assertEqual(priv(player).pausePosition, 15, 'pause anchor is the seek target');
+    assertEqual(priv(player).streamingStarted, false, 'streaming flagged not-started for a clean retry');
+});
+
+// --- AC6 retry contract: same-target seek after recovery refetches -------------------------
+
+// After recoverFromFailedRefill the scheduler is empty (clearForSeek was called). A seek to
+// the SAME position (seekPosition == playbackOffset) must route to seekBeyondBuffer — not
+// seekWithinBuffer, which would be a silent no-op against the degenerate [P,P] empty window.
+test('same-target seek after recovery routes to seekBeyondBuffer (AC6 retry)', () => {
+    const player = makePlayer();
+    const wav = new FakeStreamDecoder(true, 1000);
+    // Start with a populated window [30, 60), then simulate recovery at position 15:
+    // clearForSeek empties the scheduler; setPlaybackOffset anchors it to 15.
+    const scheduler = new FakeScheduler(30, 30);
+    arm(player, { scheduler, duration: 120, streamDecoder: wav });
+    // Drive recovery state manually (the same state recoverFromFailedRefill leaves).
+    player.recoverFromFailedRefill(15);
+    // At this point: scheduler.hasBuffers() == false, playbackOffset == 15, totalDuration == 0.
+    // A seek to 15 (the recovery anchor) must refetch, not silently resolve from the empty window.
+    const result = player.seek(15);
+    assertEqual(result.success, true, 'seek succeeds after recovery');
+    assertEqual(result.seekBeyondBuffer, true, 'same-target seek after recovery signals a refetch (AC6 retry)');
+    assertEqual(wav.requestedOffsetFor, 15, 'WAV resolver used for the retry offset');
+});
+
+// AC4 not regressed: a seek within a POPULATED retained window still resolves from buffer.
+// This is the same test as the existing AC4 test but named explicitly to confirm the
+// hasBuffers() guard does not affect the populated case.
+test('seek within populated retained window still resolves in-buffer — AC4 not regressed', () => {
+    const player = makePlayer();
+    // Populated window [30, 60) — hasBuffers() starts true (total=30 > 0).
+    const scheduler = new FakeScheduler(30, 30);
+    arm(player, { scheduler, duration: 120, streamDecoder: new FakeStreamDecoder(true, 1000) });
+
+    const result = player.seek(45); // inside [30, 60)
+    assertEqual(result.success, true, 'seek succeeds');
+    assertEqual(result.seekBeyondBuffer ?? false, false, 'populated in-window seek does NOT signal a refetch');
+    assertEqual(scheduler.clearedForSeek, false, 'scheduler not cleared for an in-buffer seek (no refetch)');
+});
+
+// --- run -------------------------------------------------------------------------------------
+if (failures.length > 0) {
+    console.error(failures.join('\n'));
+    console.error(`\n${failures.length} FAILED, ${passed} passed`);
+    process.exit(1);
+} else {
+    console.log(`ALL ${passed} TESTS PASSED`);
+}

DeepDrftPublic/Interop/audio/AudioPlayer.ts

@@ -432,18 +432,36 @@ export class AudioPlayer {
             return { success: false, error: 'Invalid seek position' };
         }
 
+        // bufferStart is the absolute track time at which buffers[0] begins. Under Phase 21.1
+        // partial eviction this is the start of the RETAINED BACK-WINDOW TAIL — eviction advances
+        // playbackOffset as it drops played buffers off the front — so [bufferStart, bufferEnd] is
+        // exactly the window currently held in memory.
         const bufferStart = this.scheduler.getPlaybackOffset();
         const bufferEnd = this.scheduler.getTotalDuration() + bufferStart;
 
-        // Position must be within [bufferStart, bufferEnd] to use buffered content.
-        // A lower-bound check is required: after a seek-beyond-buffer, bufferStart is
-        // set to the prior seek position. Seeking to a position below bufferStart would
-        // produce a negative bufferRelativePosition in seekWithinBuffer, silently
-        // clamping to position 0 of the offset buffer instead of the requested time.
-        if (position >= bufferStart && position <= bufferEnd) {
+        // The window-miss test for BOTH directions, and the 21.3 refill trigger for backward seeks.
+        // Position must be within [bufferStart, bufferEnd] AND the scheduler must hold buffers to
+        // resolve from the retained window:
+        //   - position >= bufferStart AND hasBuffers : UC3 — seek back within the retained back-window.
+        //     Served from buffer with NO network refetch. (The lower bound is load-bearing: after
+        //     eviction or a prior seek-beyond-buffer, bufferStart > 0, and a target below it would
+        //     otherwise produce a negative bufferRelativePosition in seekWithinBuffer, silently clamping
+        //     to position 0.)
+        //   - position < bufferStart   : UC4 — seek back PAST the retained tail (the window was evicted).
+        //     Falls through to seekBeyondBuffer, which is the existing Range path run toward an EARLIER
+        //     offset. This is the 21.3 window-miss refill: "a seek the listener didn't initiate" reuses
+        //     the same per-path resolver + reinit a forward seek-beyond-buffer uses, no new mechanism.
+        //   - position > bufferEnd     : UC2/UC5 — forward seek beyond buffer, unchanged.
+        //   - !hasBuffers (degenerate [P,P] window post-recovery): the window check above would
+        //     spuriously route ANY target to seekWithinBuffer (bufferStart==bufferEnd==seekPosition
+        //     after recoverFromFailedRefill). Force seekBeyondBuffer so a same-target retry actually
+        //     refetches (AC6 retry contract). The !hasBuffers guard only fires in the degenerate case —
+        //     a populated retained window has buffers and is unaffected (AC4 not regressed).
+        if (position >= bufferStart && position <= bufferEnd && this.scheduler.hasBuffers()) {
             return this.seekWithinBuffer(position);
         } else {
-            // Seeking outside buffered window - signal C# to fetch new stream
+            // Seeking outside the retained window, or to any position in an empty scheduler —
+            // signal C# to fetch a new stream from the resolved offset.
             return this.seekBeyondBuffer(position);
         }
     }
@@ -580,6 +598,44 @@ export class AudioPlayer {
         }
     }
 
+    /**
+     * Recover the player into a clean, paused-but-loaded state after a window-miss REFILL failed
+     * (Phase 21.3 / AC6). A refill is "a seek the listener didn't initiate"; when its Range fetch or
+     * reinit fails mid-stream, the pre-seek loop has already been cancelled and drained, but the
+     * scheduler is still holding stale pre-seek buffers and is still `isActive_`. Left alone it would
+     * play the retained tail to exhaustion and fire `onPlaybackEnded` — a SILENT FALSE END (the
+     * "wedged playing with a starved scheduler" AC6 forbids).
+     *
+     * The recovery mirrors `PlaybackScheduler.playFromPosition`'s end-of-buffer recovery in spirit:
+     * stop pretending to play. We stop all sources and clear the buffers for a seek (clearForSeek
+     * keeps no stale audio but is ready to accept a fresh continuation), set the offset to the
+     * requested seek position, and leave the player paused there. The track stays loaded so the
+     * listener can retry the seek or pick another track — no new transport control, only a recoverable
+     * stop (C4). A subsequent seek to the same target re-enters seekBeyondBuffer cleanly because the
+     * offset names the seek position and the scheduler is empty (so it routes to a fresh fetch).
+     *
+     * @param seekPosition The seek target the failed refill was aiming for; becomes the resume anchor.
+     */
+    recoverFromFailedRefill(seekPosition: number): AudioResult {
+        try {
+            this.stopProgressTracking();
+            // Halt the starved scheduler and drop the stale pre-seek buffers so no false end can fire.
+            this.scheduler.clearForSeek();
+            this.scheduler.setPlaybackOffset(seekPosition);
+
+            // Paused-but-loaded: not playing, not mid-seek-stream. pausePosition anchors a retry.
+            this.isPlaying = false;
+            this.isPaused = true;
+            this.pausePosition = seekPosition;
+            this.streamingStarted = false;
+            this.streamingCompleted = false;
+
+            return { success: true };
+        } catch (error) {
+            return { success: false, error: (error as Error).message };
+        }
+    }
+
     // ==================== Volume ====================
 
     setVolume(volume: number): AudioResult {

DeepDrftPublic/Interop/audio/index.ts

@@ -131,6 +131,15 @@ const DeepDrftAudio = {
         return player.reinitializeFromOffset(totalStreamLength, seekPosition);
     },
 
+    // Phase 21.3 / AC6: recover into a clean paused-but-loaded state after a window-miss refill
+    // (seek-back past the retained tail) failed its Range fetch or reinit. Prevents the starved
+    // scheduler from firing a silent false end; leaves the track loaded so a retry is possible.
+    recoverFromFailedRefill: (playerId: string, seekPosition: number): AudioResult => {
+        const player = audioPlayers.get(playerId);
+        if (!player) return { success: false, error: 'Player not found' };
+        return player.recoverFromFailedRefill(seekPosition);
+    },
+
     setVolume: (playerId: string, volume: number): AudioResult => {
         const player = audioPlayers.get(playerId);
         if (!player) return { success: false, error: 'Player not found' };