From b11188e9dd15aa574ec2c15c3444986f88694b70 Mon Sep 17 00:00:00 2001
From: Duncan Tourolle <duncan@tourolle.paris>
Date: Tue, 28 Jul 2026 23:03:17 +0200
Subject: [PATCH] docs(player): backend unification findings + correct false
 parity claims

Investigation into unifying the playback backends (Linux/MPV, Android/ExoPlayer,
Windows/webview) onto one engine with hardware acceleration. Conclusion: video
cannot be unified onto a native engine; audio can.

The blocker is not mpv-specific. WebKitGTK, WebView2 and Android WebView each
draw into their own compositor surface, so a native video surface sits either
entirely above or entirely below the webview and cannot interleave with HTML.
GStreamer and libVLC fail identically. mpv would additionally regress streaming:
it has no adaptive bitrate, while the current hls.js path does.

Six specs added:
- playback-backend-unification: the analysis and decision, with evidence
- android-audio-settings-parity: set_audio_settings on ExoPlayerBackend
- android-native-video-spike: timeboxed test of SurfaceView compositing
- windows-native-audio-backend: replace the webview <audio> shim with libmpv
- libmpv2-migration: dead libmpv git pin -> libmpv2, plus a LICENSE file
- playback-docs-corrections: the requirement-status fixes applied here

Corrections to requirements.md, all verified against source:
- UR-031/DR-034 claimed crossfade was "Done (Linux only)". It is implemented
  nowhere (mpv_backend.rs has a bare TODO) and is architecturally blocked on
  mpv, whose single-stream audio chain cannot feed acrossfade's two inputs.
- Parity matrix listed crossfade as a Linux/Android gap; it is neither.
- The matrix omitted the equalizer, which has the same Linux-only shape.
- The suggested ConcatenatingMediaSource is deprecated in current Media3.

nativeAdapter.ts cited tauri#10152 as an upstream blocker for native Android
video. That issue is a stale feature request, dead since 2024-07-01; the
capability shipped in tauri 27d01834 (2024-09-02), and the related
black-screen bug was fixed in wry 0.39.4 (we ship 0.55.x). What is genuinely
unproven is SurfaceView-behind-WebView compositing, which the spike now tracks.
---
 docs/requirements.md                        |  38 ++--
 docs/specs/android-audio-settings-parity.md | 209 ++++++++++++++++++
 docs/specs/android-native-video-spike.md    | 196 +++++++++++++++++
 docs/specs/audio-equalizer.md               |   5 +
 docs/specs/libmpv2-migration.md             | 169 +++++++++++++++
 docs/specs/playback-backend-unification.md  | 226 ++++++++++++++++++++
 docs/specs/playback-docs-corrections.md     | 198 +++++++++++++++++
 docs/specs/windows-native-audio-backend.md  | 209 ++++++++++++++++++
 src/lib/player/adapters/nativeAdapter.ts    |  18 +-
 9 files changed, 1251 insertions(+), 17 deletions(-)
 create mode 100644 docs/specs/android-audio-settings-parity.md
 create mode 100644 docs/specs/android-native-video-spike.md
 create mode 100644 docs/specs/libmpv2-migration.md
 create mode 100644 docs/specs/playback-backend-unification.md
 create mode 100644 docs/specs/playback-docs-corrections.md
 create mode 100644 docs/specs/windows-native-audio-backend.md

diff --git a/docs/requirements.md b/docs/requirements.md
index d23fb9ba..8055f6ff 100644
--- a/docs/requirements.md
+++ b/docs/requirements.md
@@ -41,7 +41,7 @@ For a narrative overview of the system design, see
 | UR-028 | Navigate to artist/album by tapping names in now playing view | High | Done |
 | UR-029 | Toggle between grid and list view in library | Medium | Done |
 | UR-030 | Quick genre browsing and filtering | Medium | Done |
-| UR-031 | Crossfade between audio tracks | Low | Done (Linux only) |
+| UR-031 | Crossfade between audio tracks | Low | Not implemented (blocked — see DR-034) |
 | UR-032 | Gapless playback for seamless album listening | Medium | Done (Linux only) |
 | UR-033 | Volume normalization to prevent volume jumps between tracks | Low | Done (Linux only) |
 | UR-034 | Rich home screen with hero banners, carousels, and personalized sections | High | Done |
@@ -193,7 +193,7 @@ Internal architecture, components, and application logic.
 | DR-031 | Clickable artist/album links in now playing view | UI | UR-028 | Done |
 | DR-032 | List view option for library browsing (albums, artists) | UI | UR-029 | Done |
 | DR-033 | Genre browsing screen with quick filters | UI | UR-030 | Done |
-| DR-034 | Crossfade engine with configurable duration (0-12s) | Player | UR-031 | Done (Linux only) |
+| DR-034 | Crossfade engine with configurable duration (0-12s) | Player | UR-031 | Not implemented (blocked on MPV: single-stream audio chain; `acrossfade` needs 2 inputs — see docs/specs/playback-backend-unification.md) |
 | DR-035 | Gapless playback between sequential tracks | Player | UR-032 | Done (Linux only) |
 | DR-036 | Volume normalization with preset levels (Loud/Normal/Quiet) | Player | UR-033 | Done (Linux only) |
 | DR-037 | Remote session browser and control UI | UI | UR-010 | Done |
@@ -499,22 +499,36 @@ The `PlayerBackend` trait defines optional audio settings methods with default e
 | Basic playback | ✅ | ✅ | Parity |
 | Volume control | ✅ | ✅ | Parity |
 | Seek | ✅ | ✅ | Parity |
-| Crossfade | ✅ | ❌ | Gap |
-| Gapless playback | ✅ | ❌ | Gap |
-| Volume normalization | ✅ | ❌ | Gap |
+| Crossfade | ❌ | ❌ | Not implemented (blocked on MPV) |
+| Gapless playback | ✅ | ⚠️ | Implemented, pending on-device verification |
+| Volume normalization | ✅ | ⚠️ | Implemented (LoudnessEnhancer — gain stage, approximate vs MPV's dynaudnorm), pending on-device verification |
+| Equalizer (10-band) | ✅ | ⚠️ | Implemented (resampled onto device bands), pending on-device verification |
 | Position updates | 250ms | On-demand | Inconsistent |
 
-**Future Fix**:
-1. Implement `set_audio_settings()` in `ExoPlayerBackend`
-2. Add Kotlin-side ExoPlayer configuration for crossfade (using `ConcatenatingMediaSource` or `DefaultMediaSourceFactory`)
-3. Implement gapless via ExoPlayer's built-in gapless support
-4. Add volume normalization via ExoPlayer's `LoudnessEnhancer` or audio processor
-5. Standardize position update frequency across platforms
+**Status** (see docs/specs/android-audio-settings-parity.md):
+1. ✅ `set_audio_settings()` implemented in `ExoPlayerBackend` (JSON over JNI)
+2. ✅ Gapless via ExoPlayer's `pauseAtEndOfMediaItems`
+3. ✅ Volume normalization via `LoudnessEnhancer`
+4. ✅ Equalizer via `android.media.audiofx.Equalizer`, canonical 10 bands
+   resampled onto the device's band centres
+5. ⬜ **Not yet verified on a physical device** — the EQ/normalization effects
+   depend on device-specific `AudioEffect` availability and band layouts
+6. ⬜ Flip the trait's `set_audio_settings` default from `Ok(())` to
+   `Err(not_implemented())` so a backend that omits it fails loudly instead of
+   silently reporting success. Deferred until (5) confirms the Android path works
+7. ⬜ Standardize position update frequency across platforms
+
+Crossfade is deliberately absent: it is unimplemented on every platform and
+architecturally blocked on MPV, so building it on Android alone would invert the
+parity gap. (The previously suggested `ConcatenatingMediaSource` is also
+deprecated in current Media3.)
 
 **Impact**:
 - Medium - Android users lack audio enhancement features advertised in requirements
 - User experience differs between platforms
-- UR-031 (Crossfade), UR-032 (Gapless), UR-033 (Normalization) only work on Linux
+- UR-032 (Gapless), UR-033 (Normalization) and UR-027 (Equalizer) are now
+  implemented on Android as well as Linux, pending on-device verification
+- UR-031 (Crossfade) works nowhere — see DR-034
 
 **Traces To**: IR-004, UR-031, UR-032, UR-033, DR-034, DR-035, DR-036
 
diff --git a/docs/specs/android-audio-settings-parity.md b/docs/specs/android-audio-settings-parity.md
new file mode 100644
index 00000000..a7136ff3
--- /dev/null
+++ b/docs/specs/android-audio-settings-parity.md
@@ -0,0 +1,209 @@
+# Spec: Android audio settings parity (EQ, normalization, gapless)
+
+**Status:** Proposed
+**Requirements:** UR-031, UR-032, UR-033, UR-027 → DR-034, DR-035, DR-036, DR-030; IR-004
+**UX spec:** n/a — no UI change; Settings › Audio already renders these controls
+**Supersedes / revises:** closes the audio half of the parity gap recorded in [playback-backend-unification.md](playback-backend-unification.md)
+
+## Summary
+
+Implement `set_audio_settings` / `audio_settings` on `ExoPlayerBackend` so the
+equalizer, volume normalization, and gapless playback settings actually take
+effect on Android. Today the Settings › Audio panel renders these controls on
+Android and they silently do nothing — `ExoPlayerBackend` is the only backend
+that does not override the trait's no-op defaults.
+
+Crossfade is explicitly **not** included; see Out of scope.
+
+## Motivation
+
+`PlayerBackend` declares `set_audio_settings` with a default `Ok(())` body.
+`MpvBackend`, `NullBackend`, and `WebviewAudioBackend` all override it;
+`ExoPlayerBackend` does not. The settings are persisted, pushed to the backend on
+every track load, and displayed in the UI — and then dropped on the floor.
+
+This is the single most user-visible platform divergence in the app: a user who
+sets a "Rock" EQ preset on Android sees the sliders move and hears no change.
+
+The backend-unification investigation ruled out fixing this by swapping engines
+(video cannot be unified; see the sibling spec), so the fix is to implement the
+trait methods where they are missing.
+
+## Layer assignment
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| Band count, centre frequencies, gain range, preset→curve map | Rust (existing) | Already domain-owned in `settings.rs` per [audio-equalizer.md](audio-equalizer.md). Android must consume the same `AudioSettings`, not define its own bands. Duplicating the band layout in Kotlin would be a taxonomy leak of exactly the kind `check:boundary` guards against. |
+| Mapping `AudioSettings` → Android audio-effect parameters | Rust → JNI boundary | Platform playback detail, the direct analogue of `build_af_filter` in `mpv_backend.rs`. Belongs with the other `set_audio_settings` code. |
+| Attaching/detaching `Equalizer` and `LoudnessEnhancer` to the ExoPlayer audio session | Kotlin (`JellyTauPlayer.kt`) | Android platform API mechanics; needs the live `audioSessionId`, which only the Kotlin layer holds. |
+| Normalization preset (Loud/Normal/Quiet) → target gain | Rust (existing) | `VolumeLevel` is domain vocabulary; the same preset must mean the same loudness on every platform. |
+| Rendering sliders / preset chips | Frontend (existing) | Pure presentation; unchanged by this spec. |
+
+Borderline row: attaching the effects could arguably be driven entirely from
+Rust via JNI property calls. It goes to Kotlin because `AudioEffect` construction
+requires the audio session id and must be re-attached when ExoPlayer rebuilds its
+audio sink — lifecycle state that lives in `JellyTauPlayer.kt`. Rust still owns
+*what* the values are; Kotlin owns *when* the effect objects exist.
+
+## Design
+
+### Rust — `ExoPlayerBackend` (`src-tauri/src/player/android/mod.rs`)
+
+Override the two defaulted methods, mirroring the shape of the existing
+`set_audio_track` JNI call:
+
+```rust
+fn set_audio_settings(&mut self, settings: &AudioSettings) -> Result<(), PlayerError> {
+    let s = settings.clone().with_crossfade_clamped().with_equalizer_normalised();
+    // Serialize as JSON — the same pattern load() already uses for subtitles,
+    // avoiding a 6-arg JNI signature that has to change every time a field lands.
+    let json = serde_json::to_string(&s).map_err(|e| PlayerError { message: e.to_string() })?;
+    // Kotlin: fun setAudioSettings(json: String)
+    self.call_player_method_string("setAudioSettings", &json)?;
+    self.shared_state.lock_safe().audio_settings = s;
+    Ok(())
+}
+
+fn audio_settings(&self) -> AudioSettings {
+    self.shared_state.lock_safe().audio_settings.clone()
+}
+```
+
+`ExoPlayerState` gains an `audio_settings: AudioSettings` field. Note
+`ExoPlayerBackend` currently holds no such state — `position`/`state`/`volume` are
+all pushed in by JNI callbacks — so this is the first *pull*-side field. That is
+correct: audio settings are commanded downward, never reported upward.
+
+### Kotlin — `JellyTauPlayer.kt`
+
+```kotlin
+fun setAudioSettings(json: String) {
+    val s = JSONObject(json)
+    applyEqualizer(s.getBoolean("equalizerEnabled"), s.getJSONArray("equalizerBands"))
+    applyNormalization(s.getBoolean("normalizeVolume"), s.getString("volumeLevel"))
+    exoPlayer.pauseAtEndOfMediaItems = !s.getBoolean("gaplessPlayback")
+}
+```
+
+Three independent mechanisms:
+
+- **Gapless** — nearly free. ExoPlayer is gapless by default for compatible
+  formats; honouring the setting means *disabling* it when the user turns it off,
+  via `pauseAtEndOfMediaItems`. Note this only applies within a loaded playlist;
+  our queue loads one item at a time, so verify behaviour before claiming DR-035
+  on Android (see Testing).
+- **Equalizer** — `android.media.audiofx.Equalizer` bound to
+  `exoPlayer.audioSessionId`. Android's EQ exposes a device-dependent band count
+  (commonly 5) at fixed centre frequencies, which will **not** match our 10-band
+  ISO layout. Rust owns the canonical 10 bands; Kotlin resamples them onto the
+  device's bands by nearest-centre-frequency interpolation. Gains are in
+  millibels (`setBandLevel` takes mB, we store dB → ×100), clamped to the
+  device's reported `getBandLevelRange()`.
+- **Normalization** — `android.media.audiofx.LoudnessEnhancer`, also bound to the
+  audio session, `setTargetGain(mB)` derived from `VolumeLevel`. This is a gain
+  booster, not a true EBU R128 normalizer like MPV's `dynaudnorm`; parity is
+  approximate and should be documented as such rather than overclaimed.
+
+Lifecycle: build the effects lazily on first use, release them in `release()`,
+and re-attach on `onAudioSessionIdChanged` — ExoPlayer can rebuild its audio sink
+(e.g. on a format change), which invalidates effects bound to the old session.
+
+### Make the silent-failure mode impossible
+
+The trait's default is the root cause of this whole class of bug:
+
+```rust
+// backend.rs:85 — reports success while doing nothing
+fn set_audio_settings(&mut self, _settings: &AudioSettings) -> Result<(), PlayerError> {
+    Ok(())
+}
+```
+
+Android inherits this, so every EQ/normalization change on Android returns `Ok`
+and silently does nothing — the UI ships and has no effect, with no error anywhere.
+
+Once `ExoPlayerBackend` implements the methods, **change the trait default to
+`Err(PlayerError::not_implemented())`**, matching how `set_audio_track` /
+`set_subtitle_track` already behave. Any future backend that forgets to implement
+audio settings then fails loudly instead of lying.
+
+Check the call sites before flipping it: `NullBackend` overrides both methods, so
+the graceful-degradation path is unaffected, but confirm nothing treats a
+`set_audio_settings` error as fatal to playback.
+
+### Re-application on track load
+
+`PlayerController` already re-pushes `AudioSettings` per track on the platforms
+that implement it; the Android path inherits that for free once the trait methods
+exist. No controller change.
+
+### 🔴 Threading note
+
+`setAudioSettings` is invoked from Rust on whatever thread the command lands on.
+`AudioEffect` construction must not happen on the ExoPlayer application thread
+from inside a player callback — that is the re-entrancy hazard CLAUDE.md warns
+about, and the same shape as the `AutoplayDecision` deadlock. Post the work to
+the player's handler rather than doing it inline in a listener.
+
+## Out of scope
+
+- **Crossfade (UR-031 / DR-034).** Not implemented on *any* platform today, and
+  architecturally blocked on MPV (single-stream audio chain; `acrossfade` needs
+  two inputs). Implementing it on Android alone would invert the parity gap. It
+  needs its own spec and probably two player instances.
+- True EBU R128 normalization. `LoudnessEnhancer` is a gain stage; matching
+  `dynaudnorm` exactly is out of reach without a custom `AudioProcessor`.
+- Windows audio settings — see [windows-native-audio-backend.md](windows-native-audio-backend.md).
+
+## Acceptance criteria
+
+- [ ] `ExoPlayerBackend` overrides `set_audio_settings` and `audio_settings`.
+- [ ] EQ preset change on Android audibly changes playback; setting persists across track changes and app restart.
+- [ ] Normalization toggle audibly changes level; the three presets are ordered Loud > Normal > Quiet.
+- [ ] Disabling gapless produces a gap between consecutive tracks; enabling it does not.
+- [ ] Effects are released on `release()` and survive an audio-session rebuild.
+- [ ] `requirements.md` parity matrix updated: EQ and normalization ✅ Android.
+- [ ] `bun run check` and `bun run test` pass.
+- [ ] `cargo fmt` clean, `cargo clippy` clean, `bun run test:rust` passes.
+- [ ] `bun run check:boundary` passes.
+- [ ] New requirement-implementing code carries `// TRACES:` comments.
+- [ ] `bindings.ts` regenerated if Rust types changed.
+
+## Testing
+
+**Rust** (`cargo test`): `set_audio_settings` stores the sanitized settings and
+`audio_settings()` returns them — assert clamping/normalisation is applied
+(crossfade clamped to 12s, band vector normalised to `EQ_BANDS.len()`). The JNI
+call itself is not unit-testable; extract the JSON serialization into a pure
+function and test that its shape matches what the Kotlin parser expects. That
+serialization contract is the part most likely to silently break.
+
+**Kotlin**: the band-resampling function (10 canonical bands → N device bands) is
+pure arithmetic — extract it and unit-test it, including the degenerate cases of
+a 5-band device and a device reporting 10 bands.
+
+**Manual, on device** (these are the ones that actually prove it):
+1. Set Bass Boost, play a track, confirm audible change.
+2. Toggle normalization mid-track; confirm level change without a playback stall.
+3. Queue two gapless-encoded tracks, toggle the setting, confirm the gap appears/disappears.
+4. Force a format change (44.1kHz → 48kHz track) and confirm the EQ still applies afterwards — this exercises the session-rebuild re-attach.
+
+## TRACES
+
+- `ExoPlayerBackend::set_audio_settings` → `// TRACES: UR-027, UR-032, UR-033 | DR-030, DR-035, DR-036`
+- Kotlin `setAudioSettings` / `applyEqualizer` / `applyNormalization` → same IDs
+- Band-resampling helper + its tests → `DR-030 | UT-xxx`
+
+## Notes for the implementer
+
+- Read [audio-equalizer.md](audio-equalizer.md) first — it defines the canonical
+  band layout and the preset→curve rule this spec consumes. Do not redefine bands
+  in Kotlin.
+- Android source edits go in `src-tauri/android/src` (canonical tree), then run
+  `scripts/sync-android-sources.sh`. Never edit the `gen/` tree.
+- There is a **stale duplicate** `JellyTauPlayer.kt` (285 lines) at
+  `src-tauri/android/app/src/main/java/com/dtourolle/jellytau/player/` alongside
+  the real 1103-line file at `src-tauri/android/src/main/java/...`. Edit the
+  latter. Consider deleting the former as a separate change.
+- A parallel Claude session may be active — `git diff` before "repairing"
+  unexpected changes.
diff --git a/docs/specs/android-native-video-spike.md b/docs/specs/android-native-video-spike.md
new file mode 100644
index 00000000..40095a7d
--- /dev/null
+++ b/docs/specs/android-native-video-spike.md
@@ -0,0 +1,196 @@
+# Spec: Android native video — transparent-webview spike
+
+**Status:** Proposed (spike — timeboxed, may conclude "not viable")
+**Requirements:** IR-004, UR-003, UR-004 → DR-001, DR-023, DR-024
+**UX spec:** n/a — no intended visual change; the video surface must land exactly where the `<video>` element is today
+**Supersedes / revises:** acts on finding 2 of [playback-backend-unification.md](playback-backend-unification.md)
+
+## Summary
+
+Test whether ExoPlayer's existing `SurfaceView` video path can be composited
+behind a transparent Tauri WebView on Android. If it works, Android regains
+hardware video decoding (MediaCodec) and libass-quality ASS/SSA subtitles, both
+of which the current webview path lacks. If it does not, we document why and
+delete the dead code.
+
+This is a **spike**, not a feature commitment. The deliverable is a yes/no answer
+with evidence, plus either a working path behind a flag or a removal.
+
+## Motivation
+
+`createAdapter()` hardcodes `const effectiveKind = "html5"` and does
+`void backendKind`, discarding the `use_html5_element` value Rust computes in
+`get_player_status`. As a result:
+
+- `NativePlayerAdapter` is dead code.
+- `JellyTauPlayer.kt`'s `getOrCreateSurfaceView()` — which already calls
+  `setZOrderMediaOverlay(false)` and wires `setVideoSurfaceHolder` — is
+  unreachable.
+- Android video decodes in the WebView instead of via MediaCodec, despite
+  `CodecDetector.kt` going to the trouble of reporting hardware codec
+  capabilities back to Rust for DeviceProfile generation.
+
+The code comment in `nativeAdapter.ts:11-14` justifies this by citing
+tauri#10152 as an upstream blocker. **That justification is stale.**
+
+### Why the blocker no longer holds
+
+- tauri#10152 is open but **dead since 2024-07-01**, and it is a *feature
+  request* ("Support transparent webviews on mobile"), not a bug report about
+  compositing.
+- The capability shipped in tauri commit `27d01834` (2024-09-02) — a clippy
+  cleanup that moved `transparent()` out of the desktop-gated impl block, fencing
+  only the tao call behind `#[cfg(desktop)]`. Because it landed as unrelated
+  cleanup, nobody closed the issue.
+- The black/white-screen reports (tauri#8381, tauri#9408) were a real but
+  *different* bug: a broken JNI signature for `setBackgroundColor`, fixed in
+  **wry 0.39.4** (PR #1237). We ship wry 0.55.x.
+- Current wry calls `setBackgroundColor(0)` unconditionally on Android when
+  transparency is requested.
+
+### The honest caveat
+
+**Nobody has demonstrated SurfaceView-behind-WebView on Tauri Android.** A search
+of both `tauri-apps/tauri` and `tauri-apps/wry` issues for `surfaceview` returns
+zero results, and the one native-video Tauri plugin
+(`YeonV/tauri-plugin-videoplayer`) sidesteps compositing by launching a separate
+fullscreen Activity. Nothing upstream blocks this; nothing upstream proves it.
+Hence: spike, not feature.
+
+Note this is the *Android* question only. The equivalent Linux compositing
+problem is maintainer-declared unfixable and is **not** in scope — see the
+unification spec.
+
+## Layer assignment
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| Which video backend this platform uses | Rust (existing) | `get_player_status` already computes `use_html5_element`. The frontend must *consume* it, not decide it. Restoring that is the point of the spike. |
+| Surface creation, z-ordering, `setVideoSurfaceHolder` lifecycle | Kotlin | Android platform mechanics; already written in `JellyTauPlayer.kt`. |
+| Seek/audio-track *strategy* | Rust (existing) | Already returned by `player_seek_video` / `player_switch_audio_track`; `NativePlayerAdapter` executes the chosen primitive. Unchanged — this is exactly what the `PlayerAdapter` contract was built for. |
+| Positioning the surface under the video viewport | Frontend | Pure presentation/layout. **This is the risk area** — see Design. |
+
+## Design
+
+### Phase 1 — prove compositing (no app changes)
+
+Before touching the adapter factory, verify the primitive works at all:
+
+1. Set `"transparent": true` in `tauri.conf.json` for the Android build, plus
+   `html, body { background: transparent; }`.
+2. Confirm the WebView is genuinely transparent (a native view behind it is
+   visible) and that the app does not regress to a black/white screen.
+
+If this fails, stop — everything downstream is moot, and the finding is that
+Tauri Android transparency is still broken in practice despite the shipped fix.
+
+### Phase 2 — un-hardcode the factory
+
+```ts
+// src/lib/player/adapters/index.ts
+export function createAdapter({ backendKind, host, bridge }: CreateAdapterArgs): PlayerAdapter {
+  return backendKind === "native"
+    ? new NativePlayerAdapter(host)
+    : new Html5PlayerAdapter(host, bridge);
+}
+```
+
+`backendKind` comes from `get_player_status` (`VideoBackend::Native` on Android).
+Gate behind a setting — `experimentalNativeVideo`, default **off** — so a broken
+spike cannot ship as a regression. Rust already owns this decision; the flag only
+suppresses it.
+
+**Also in scope: remove the user-agent sniffing in
+`src/lib/services/webviewAudio.ts:30-41`.** It re-derives which audio backend the
+platform has from `navigator.userAgent` ("matching the Rust cfg gate", per its own
+comment) — the frontend deciding a backend fact it should be told. Same root cause
+as the hardcode above, same fix: consume the value Rust already computes. Fold it
+in here rather than leaving a second, subtler copy of the bug behind. If
+`get_player_status` does not currently expose enough to cover the audio case, add
+the field — that is backend work, and correct.
+
+### Phase 3 — surface positioning
+
+The hard part, and where this most likely fails. The webview's `<video>` element
+occupies a laid-out box; the `SurfaceView` must be positioned to match it, and
+kept matched through scroll, rotation, and mini-player transitions.
+
+Approach: the video view reports its `getBoundingClientRect()` to Rust, which
+forwards the rect to Kotlin to position the `SurfaceView`. This is the same
+"faking it" technique the ecosystem uses on desktop — acceptable here *only if*
+the video is effectively fullscreen on Android, which it is in the player route.
+
+**Explicit failure criterion**: if the surface cannot be kept aligned during
+rotation or the mini-player transition without visible artefacts, the spike fails
+and we keep HTML5. Do not ship a janky native path for a codec win.
+
+### What we gain if it works
+
+- **Hardware decode via MediaCodec** — `CodecDetector.kt` already reports
+  capabilities; the DeviceProfile would finally match what actually plays.
+- **ASS/SSA subtitles** are *not* automatic. ExoPlayer cannot render them; that
+  would require libmpv, which is a separate and much larger decision (see the
+  unification spec's engine comparison). Scope this spike to hardware decode
+  only, and do not claim subtitle improvements from it.
+
+## Out of scope
+
+- Linux native video. Maintainer-declared unfixable on WebKitGTK/Wayland.
+- Replacing ExoPlayer with libmpv on Android.
+- Windows native video.
+- Removing the HTML5 path. It stays as the default and the fallback.
+
+## Acceptance criteria
+
+The spike is **complete** when one of these is true:
+
+**Success path**
+- [ ] Transparent WebView confirmed working on a physical device.
+- [ ] `experimentalNativeVideo` off → behaviour byte-identical to today.
+- [ ] `webviewAudio.ts` no longer inspects `navigator.userAgent`; the platform's audio backend is read from Rust.
+- [ ] `experimentalNativeVideo` on → video plays via ExoPlayer/MediaCodec, correctly positioned, with working seek, audio-track switch, and subtitle selection through the existing `PlayerAdapter` contract.
+- [ ] No artefacts on rotation, background/foreground, or mini-player transition.
+- [ ] `adb shell dumpsys media.metrics` (or logcat) confirms a hardware decoder is in use.
+- [ ] Measured battery/thermal or CPU improvement over the HTML5 path on the same clip.
+
+**Failure path**
+- [ ] The blocking behaviour is documented in this spec with evidence.
+- [ ] `NativePlayerAdapter` and the unreachable `SurfaceView` code are deleted, or explicitly retained with a *correct* comment.
+- [ ] `nativeAdapter.ts:11-14` no longer cites tauri#10152.
+
+Either way:
+- [ ] `bun run check`, `bun run test`, `bun run check:boundary` pass.
+- [ ] `cargo fmt` / `cargo clippy` clean; `bun run test:rust` passes.
+
+## Testing
+
+Adapter-selection logic is pure and testable without a device: assert
+`createAdapter` returns `NativePlayerAdapter` for `backendKind: "native"` with
+the flag on, and `Html5PlayerAdapter` in every other combination — including that
+the flag off forces HTML5 even when Rust says native. That last case is the
+regression guard.
+
+Everything else is manual on-device; there is no meaningful way to unit-test
+surface compositing. Test on at least two devices — compositing behaviour varies
+by OEM and Android version.
+
+Per CLAUDE.md, if the spike turns into a bug fix (e.g. seek breaks under the
+native adapter), write the failing test first.
+
+## TRACES
+
+- `createAdapter` → `// TRACES: UR-003, UR-004 | DR-023, DR-024`
+- Adapter-selection tests → `UT-xxx`
+- No new requirement IDs; this spike either satisfies existing IR-004 expectations or documents why it cannot.
+
+## Notes for the implementer
+
+- **Do not skip Phase 1.** If transparency does not work, phases 2 and 3 are
+  wasted effort.
+- `VideoPlayer.svelte` has a documented hazard: no lifecycle calls after an
+  `await` in `onMount` — it flips to HTML5 mode and breaks Android seek. The
+  adapter swap touches exactly this code path.
+- tauri-specta tagged responses keep Rust field names (`new_url`, not `newUrl`).
+- Android source edits go in `src-tauri/android/src`, then run
+  `scripts/sync-android-sources.sh`.
+- A parallel Claude session may be active — `git diff` first.
diff --git a/docs/specs/audio-equalizer.md b/docs/specs/audio-equalizer.md
index 86f0e6ac..372dd49d 100644
--- a/docs/specs/audio-equalizer.md
+++ b/docs/specs/audio-equalizer.md
@@ -4,6 +4,7 @@
 **Requirements:** UR-027 → DR-030 (EQ UI), IR-020 (MPV EQ integration).
 **UX spec:** n/a (extends the Settings › Audio section, ux-flows §8.1 instant-apply).
 **Supersedes / revises:** —
+**Revised by:** [android-audio-settings-parity.md](android-audio-settings-parity.md) — lifts the "Android is a no-op" limitation below.
 
 ## Summary
 
@@ -115,6 +116,10 @@ fields ride along. `NullBackend`/Android inherit the trait default (no-op).
 ## Out of scope
 
 - Android/ExoPlayer EQ (parity gap tracked with crossfade/gapless/normalize).
+  **Now specified in [android-audio-settings-parity.md](android-audio-settings-parity.md)**,
+  which implements `set_audio_settings` on `ExoPlayerBackend`. The canonical band
+  layout and preset→curve map defined here remain authoritative; the Android side
+  resamples those bands onto the device equalizer rather than defining its own.
 - Per-track or per-library EQ profiles — one global profile only.
 - Automatic loudness/room correction; only manual bands + presets.
 - Changing the crossfade/normalize TODOs in `set_audio_settings` beyond wiring
diff --git a/docs/specs/libmpv2-migration.md b/docs/specs/libmpv2-migration.md
new file mode 100644
index 00000000..73a60818
--- /dev/null
+++ b/docs/specs/libmpv2-migration.md
@@ -0,0 +1,169 @@
+# Spec: Migrate to libmpv2 and declare the project licence
+
+**Status:** Proposed
+**Requirements:** UR-003 → IR-003 (revises the MPV integration); no new user-facing behaviour
+**UX spec:** n/a
+**Supersedes / revises:** dependency and licensing housekeeping identified in [playback-backend-unification.md](playback-backend-unification.md)
+
+## Summary
+
+Two related pieces of housekeeping that block or complicate later work:
+
+1. Replace the abandoned `libmpv` crate (pinned to a git branch) with the
+   maintained `libmpv2`.
+2. Add a `LICENSE` file. The project has none, which leaves its legal status
+   undefined while it links GPL-licensed libmpv.
+
+Neither changes user-visible behaviour. Both are prerequisites for
+[windows-native-audio-backend.md](windows-native-audio-backend.md).
+
+## Motivation
+
+### The dependency is dead
+
+```toml
+# src-tauri/Cargo.toml
+libmpv = { git = "https://github.com/ParadoxSpiral/libmpv-rs.git", branch = "master" }
+```
+
+- crates.io `libmpv` 2.0.1 was published **2020-09-29**.
+- The upstream repo's last commit was **2023-01-08**; nothing since was released.
+- We pin a git *branch*, so builds are not reproducible — the same lockfile-less
+  checkout can resolve differently over time, and CI has no protection if the
+  branch moves or the repo disappears.
+
+`libmpv2` (kohsine/libmpv2-rs) is a maintained fork of exactly this crate:
+6.0.0 released **2026-05-12**, ~23.5k recent downloads against the original's
+~1.1k, releases roughly quarterly since 2024.
+
+### The project has no licence
+
+There is no `LICENSE`/`COPYING` file and `src-tauri/Cargo.toml` has no `license`
+field. The project is open source and will never be commercial, so this is purely
+an omission — but it matters because we link libmpv, and "no licence" defaults to
+*all rights reserved*, which is incompatible with distributing a GPL-derived
+work.
+
+## Design
+
+### Part 1 — licence
+
+**Use GPLv3.** This is forced, not chosen:
+
+- mpv's default build is **GPLv2-or-later**, so the combined work must be
+  GPL-compatible.
+- Apache-2.0 is **GPLv2-incompatible** (patent-termination and indemnification
+  clauses) but GPLv3-compatible.
+- A scan of the dependency tree found Apache-2.0-**only** crates with no
+  alternative arm — most importantly **`tao`** (Tauri's own windowing crate),
+  plus `sync_wrapper`, `gethostname`, and `ring` (Apache-2.0 AND ISC).
+
+`tao` is unavoidable in a Tauri app, so GPLv2 is unavailable. Exercising mpv's
+"or later" option puts the combination at **GPLv3**.
+
+Actions:
+- Add `LICENSE` containing the GPLv3 text.
+- Add `license = "GPL-3.0-or-later"` to `src-tauri/Cargo.toml` and `license` to
+  `package.json`.
+- Note in the README that the binary links libmpv (GPLv2+) and FFmpeg.
+
+Because the project is open source, we use mpv's **default GPL build** — no
+`-Dgpl=false`, no LGPL FFmpeg build, and none of the LGPL §6 relinking analysis
+that a proprietary app would need. We keep VAAPI/VDPAU/X11 and every GPL FFmpeg
+filter.
+
+🔴 Never build FFmpeg with `--enable-nonfree` — that produces a binary that is
+**unredistributable under any licence**, open source or not.
+
+### Part 2 — libmpv → libmpv2
+
+```toml
+# Linux (and later Windows, per the Windows audio spec)
+libmpv2 = "=6.0.0"
+```
+
+Pin exactly: `libmpv2` has broken its API in **every** major release.
+
+Breaking changes to expect, from the changelog:
+
+| Version | Change | Impact here |
+|---|---|---|
+| 4.0.0 | Removed command helper methods — call `mpv.command(...)` directly | Low; we already use `command`/`set_property` |
+| 5.0.0 | Removed `mpv_node` support entirely (properties return strings; parse JSON yourself); `EventContext` folded into `Mpv`; `ProtocolContext` → `Protocol` | **Medium** — `start_event_loop` uses `create_event_context()`; check whether that call still exists |
+| 6.0.0 | `RenderContext::new()` → `Mpv::create_render_context()`; `'static` bound on `OpenGLInitParams`; render context now borrows `Mpv` (fixes a use-after-free) | **None** — we do not use the render API |
+
+The last row matters: we run mpv audio-only (`video = no`), so the entire render
+surface is irrelevant to us. Consider disabling the default `render` feature to
+reduce build surface.
+
+The main porting work is the event loop in `mpv_backend.rs` — `wait_event`,
+`disable_deprecated_events`, and the `FileLoaded` / `PlaybackRestart` /
+`PropertyChange` / `EndFile` handling, given 5.0.0 folded `EventContext` into
+`Mpv`.
+
+Everything else — `set_property` calls, the `af` filter graph, the 250ms position
+thread, the seek-suppression window — should port unchanged.
+
+## Layer assignment
+
+No logic moves. This is a dependency swap plus a licence file; the
+`PlayerBackend` trait boundary is untouched.
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| mpv event → `PlayerStatusEvent` mapping | Rust (unchanged) | Already correct; only the binding API beneath it changes. |
+
+## Out of scope
+
+- Any behaviour change. If playback behaves differently after this, that is a bug.
+- Windows support — separate spec, but this must land first.
+- Adopting the render API. We are audio-only on mpv.
+- Re-licensing decisions beyond adding the file the project already implies.
+
+## Acceptance criteria
+
+- [ ] `LICENSE` (GPLv3) present; `license` field set in `Cargo.toml` and `package.json`.
+- [ ] A full dependency-licence audit has been run (`cargo install cargo-license && cargo license`) and confirms no GPLv3-incompatible dependency. *(The scan behind this spec resolved 441 of 575 crates from the local registry cache; the remaining 134 are unverified.)*
+- [ ] `libmpv` git dependency removed; `libmpv2` pinned to an exact version.
+- [ ] Linux audio playback works identically: play/pause/seek/volume, queue advance, gapless, EQ, normalization, sleep timer.
+- [ ] Position updates still arrive at 250ms; the 150ms post-seek suppression still prevents the jump-to-zero glitch.
+- [ ] `EndFile` still emits `PlaybackEnded` only for EOF (not STOP/QUIT/ERROR) — autoplay depends on this.
+- [ ] Builder image updated if the libmpv dev package requirement changed; **no toolchain install added to any CI step**.
+- [ ] `bun run check`, `bun run test`, `bun run check:boundary` pass.
+- [ ] `cargo fmt` clean, `cargo clippy` clean, `bun run test:rust` passes.
+
+## Testing
+
+The existing `mpv_backend_test.rs` plus the `build_af_filter`,
+`eq_filter_entries`, and `normalize_filter_entry` tests are the regression net —
+they must pass unchanged, since none of them touch the binding API.
+
+The event loop has no unit tests and is where the risk concentrates. Verify
+manually on Linux:
+
+1. Play → pause → play; confirm position does not flash to 0:00 (the known
+   playing-event regression).
+2. Seek mid-track; confirm no jump-to-zero within 150ms.
+3. Let a track end naturally; confirm autoplay advances (exercises `EndFile` EOF).
+4. Press stop; confirm autoplay does **not** advance.
+5. Sleep-timer expiry; confirm it stops without triggering autoplay.
+
+Cases 3–5 are the ones most likely to break silently, and each corresponds to a
+bug already fixed once in this codebase.
+
+## TRACES
+
+- `MpvBackend` construction / event loop → existing `// TRACES: UR-003 | IR-003`, unchanged
+- No new requirement IDs; this is a dependency migration.
+
+## Notes for the implementer
+
+- Do this **before** the Windows audio backend.
+- Read the 4.0/5.0/6.0 changelogs before writing code — the crate has broken API
+  in every major release, most recently two months before this spec.
+- The crates.io `repository` field for `libmpv2` points at `kohsine/libmpv-rs`,
+  but the repo was renamed to **`libmpv2-rs`**; the old raw URLs 404.
+- `libmpv2-sys` ships pregenerated bindings and vendored headers, so no libclang
+  is needed at build time — relevant to keeping the builder image thin.
+- A parallel Claude session may be active — `git diff` before "repairing"
+  unexpected changes.
diff --git a/docs/specs/playback-backend-unification.md b/docs/specs/playback-backend-unification.md
new file mode 100644
index 00000000..7cb26241
--- /dev/null
+++ b/docs/specs/playback-backend-unification.md
@@ -0,0 +1,226 @@
+# Spec: Playback backend unification — findings and strategy
+
+**Status:** Accepted (analysis; no code changes)
+**Requirements:** IR-004, UR-031, UR-032, UR-033 — revises the "Platform Playback Backend Parity" issue in requirements.md
+**UX spec:** n/a
+**Supersedes / revises:** informs [android-native-video-spike.md](android-native-video-spike.md), [android-audio-settings-parity.md](android-audio-settings-parity.md), [windows-native-audio-backend.md](windows-native-audio-backend.md)
+
+## Summary
+
+This spec records the outcome of an investigation into unifying JellyTau's
+playback backends (Linux/MPV, Android/ExoPlayer, Windows/webview) onto a single
+engine with hardware acceleration everywhere. **The conclusion is that video
+cannot be unified onto a native engine, and should not be attempted.** Audio
+*can* be, and that is where the remaining specs direct effort.
+
+No code changes follow from this spec directly. It exists so the decision is
+written down with its evidence, and so a future session does not re-run the same
+investigation.
+
+## Motivation
+
+The requirements doc carries a "Platform Playback Backend Parity" issue noting
+that audio settings work on Linux but not Android, and proposing eventual
+convergence. The natural next question — "should we just run one engine
+everywhere?" — needed answering before spending effort on per-backend patches.
+
+The investigation also surfaced that several statements in requirements.md and in
+code comments are factually wrong. Those corrections are part of the deliverable.
+
+## Findings
+
+### 1. The current architecture is not what the docs describe
+
+| Platform | Audio | Video |
+|----------|-------|-------|
+| Linux | MPV (native, **audio-only**) | webview `<video>` + hls.js |
+| Android | ExoPlayer (native) | **webview `<video>` + hls.js** |
+| Windows | webview `<audio>` | webview `<video>` + hls.js |
+
+Two surprises:
+
+- **MPV never decodes video.** `mpv_backend.rs` sets `video = no` and
+  `audio-display = no` at construction. Linux video has always been the webview.
+  Correspondingly, `player_play_item` deliberately does *not* load into MPV on
+  Linux (it calls `set_current_item`, which only updates the queue).
+- **Android video is also the webview.** `createAdapter()` in
+  `src/lib/player/adapters/index.ts` hardcodes `const effectiveKind = "html5"`
+  and does `void backendKind`, discarding the `use_html5_element` signal that
+  `get_player_status` computes in Rust. `NativePlayerAdapter` is dead code, and
+  ExoPlayer's `SurfaceView` path in `JellyTauPlayer.kt` is unreachable.
+
+So video is *already* unified — on HTML5, everywhere, by accident of that
+hardcode — and on the path without hardware decoding on Android.
+
+### 2. Native video cannot be composited with a Tauri webview
+
+This is the load-bearing finding. It is **not** an mpv limitation; it defeats
+every candidate engine identically:
+
+- **mpv**: `tauri-plugin-libmpv`'s own platform table reads Linux ⚠️
+  *"Experimental. Window embedding is not working."*
+- **GStreamer** (wry discussion #284, 2024): *"Gstreamer was rendering above the
+  surface and covering all html elements."*
+- **libVLC** (tauri discussion #6343, 2024): *"I had to render the webview in a
+  child window though because vlc kept rendering on top of it."*
+
+Root cause, from Tauri maintainer amrbashir (tauri#9220, 2024-03-30):
+
+> "we are limited to using Webkit2GTK on Linux and that requires a GTK window.
+> While possible to add a GTK widget as a child X11 window inside raw X11 window,
+> this is however a bit hacky and **it is not possible on Wayland at all**."
+
+WebKitGTK, WebView2, and Android WebView each draw into their own compositor
+surface. A native video surface is either entirely above or entirely below the
+webview; it cannot interleave with HTML. Every working example in the ecosystem
+is the same hack — a separate child window position-synced to a
+`getBoundingClientRect()` div — which breaks on resize, scroll, and any UI drawn
+over the video. For JellyTau that means the controls, subtitle overlay, and
+mini-player.
+
+The most recent comment on tauri#6343 (2026-05-23) confirms it is still unsolved:
+
+> "I'm faking it and the window is not truly embedded, basically when the parent
+> moves or resizes I reset the position and size of the libmpv window to align it
+> with an HTML div."
+
+**The principle to carry forward: audio can unify on a native engine; video
+cannot, because video needs a surface and the webview owns the surface.**
+
+### 3. mpv would regress streaming quality
+
+mpv has **no adaptive bitrate**. It delegates HLS to FFmpeg's demuxer, which
+selects one variant at open time and never adapts; mpv#3548 (2016) requested ABR
+and it never landed. `--hls-bitrate` is a static picker defaulting to `max`.
+
+The webview path already has real ABR via hls.js. Moving video to mpv would be a
+**downgrade** on every platform — no graceful degradation on weak networks, and
+quality changes requiring teardown and reload.
+
+### 4. Crossfade is architecturally blocked on mpv
+
+mpv's audio chain is single-stream. FFmpeg's `acrossfade` is an `N→A` filter
+requiring two input streams, so there is no second input to feed it. Real
+crossfade needs **two libmpv instances** with manually ramped volumes. Upstream
+maintainer response (mpv#4512, closed three minutes after opening):
+
+> "No. I also find crossfading stupid and complex, so the likeliness of that
+> happening is low."
+
+GStreamer *could* do it via `audiomixer`. mpv cannot, at any reasonable cost.
+
+### 5. Engine comparison summary
+
+| Criterion | mpv | GStreamer | libVLC |
+|-----------|-----|-----------|--------|
+| Webview compositing | ❌ Linux broken | ❌ same wall | ❌ same wall |
+| Adaptive bitrate HLS | ❌ none | ✅ adaptivedemux2 | ✅ adaptive module |
+| Rust bindings | ⚠️ `libmpv2` active; our pin is dead | ✅ `gstreamer-rs` excellent | ❌ `vlc-rs` abandoned (2018) |
+| Windows cross-MSVC | ⚠️ prebuilt DLL | ❌ pkg-config vs cargo-xwin | ❌ no better |
+| Android packaging | ✅ Maven AAR (used by Findroid) | ⚠️ Cerbero/NDK, painful | ✅ mature AAR |
+| ASS/SSA subtitles | ✅ libass built in | ✅ libass | ✅ libass |
+| Crossfade | ❌ impossible | ✅ `audiomixer` | ⚠️ unclear |
+
+Every candidate fails the first row, which is the disqualifying one.
+
+### 6. Two further options ruled out
+
+**Webview `<audio>`/`<video>` everywhere** (i.e. delete the native audio backends
+too) is dead on Android: `navigator.mediaSession` is *deliberately compiled out*
+of Android WebView (Chromium CL 2613133003), so lockscreen/media-notification
+control would be impossible. Chromium has also never shipped `audioTracks`. It
+remains fine for Windows *video*, which is what we already do.
+
+**FFmpeg-direct / Rust-native** (`ffmpeg-next`, `rsmpeg`, Symphonia) is not
+close: the safe bindings do not expose hardware decode at all, `ffmpeg-next` is
+self-declared maintenance-only, and Symphonia lacks HE-AAC and gapless AAC. This
+is a multi-person-year path to reach parity with what we already have.
+
+### 7. If libmpv is ever revisited on Android
+
+Recorded so the next investigation starts from evidence rather than repeating the
+search. The `dev.jdtech.mpv:libmpv` AAR — maintained by Findroid's author, i.e.
+another Jellyfin Android client — was inspected directly:
+
+- `libmpv.so` exports the full 54-function `mpv_*` C API with **zero `Java_`
+  symbols**; JNI is a separate optional ~19 KB `libplayer.so`. So it is drivable
+  from Rust without a Java shim. (This is precisely what disqualifies libVLC,
+  whose Android video path hard-requires a Java `AWindow` jobject.)
+- ~23 MB/ABI, versus libVLC's ~46 MB/ABI.
+- 🔴 **The published AAR is built `--enable-gpl --enable-version3` — it is
+  GPLv3**, not LGPL. Fine for us (see [libmpv2-migration.md](libmpv2-migration.md)),
+  but it would be a hard constraint for anyone shipping closed source, and an
+  LGPL rebuild would be your own build to own.
+- Top unverified risk if anyone tries this: whether `libmpv2-sys` can
+  cross-compile for `aarch64-linux-android` against that prebuilt `.so`. No
+  working example of `libmpv2` on Android was found.
+
+None of this changes the verdict — the cost is the MediaSession/foreground-service
+rewrite, not the bindings.
+
+## Decision
+
+1. **Do not unify video onto a native engine.** Video stays in the webview with
+   hls.js on all platforms. This is not a compromise — it is the configuration
+   that falls out of the compositing constraint, and it is the only one that
+   gives us ABR for free.
+2. **Android native video is worth a bounded spike anyway** — not for
+   unification, but because ExoPlayer's `SurfaceView` path already exists and
+   would restore hardware decode plus ASS/SSA subtitles. See
+   [android-native-video-spike.md](android-native-video-spike.md).
+3. **Audio parity is the real gap** and is achievable without touching any of the
+   above. See [android-audio-settings-parity.md](android-audio-settings-parity.md)
+   and [windows-native-audio-backend.md](windows-native-audio-backend.md).
+4. **Migrate the dead libmpv pin** regardless of any of this. See
+   [libmpv2-migration.md](libmpv2-migration.md).
+
+## Corrections to existing docs
+
+These are factual errors found during the investigation. Fixing them is in scope
+for this spec.
+
+| Location | Says | Actually |
+|----------|------|----------|
+| `requirements.md` UR-031 (line ~44) | "Done (Linux only)" | Not implemented on any platform. |
+| `requirements.md` DR-034 (line ~196) | "Done (Linux only)" | Not implemented anywhere — `mpv_backend.rs` has a bare `// TODO: Implement crossfade via MPV audio filters if needed`. Architecturally blocked on mpv (finding 4). |
+| `requirements.md` parity matrix | Crossfade ✅ Linux / ❌ Android | ❌ / ❌ |
+| `requirements.md` parity matrix | (no EQ row) | EQ is also Linux-only — `build_af_filter`/`eq_filter_entries` exist only in `mpv_backend.rs`. Same root cause, same fix. |
+| `nativeAdapter.ts:11-14` | Native Android video "blocked upstream by tauri#10152" | tauri#10152 is a stale *feature request*, dead since 2024-07-01. The capability shipped in tauri commit `27d01834` (2024-09-02). Not a blocker. |
+
+## Layer assignment
+
+No new logic. The one boundary observation worth recording:
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| Which video backend a platform uses (`use_html5_element`) | Rust | Already correctly computed in `get_player_status`. The frontend currently *discards* it — that is the bug, not the design. Restoring it means the frontend consumes a backend decision rather than making its own. |
+
+## Out of scope
+
+- Any code change. This spec is analysis; the sibling specs carry the work.
+- iOS/macOS. Not current targets.
+- Replacing hls.js.
+
+## Acceptance criteria
+
+- [ ] `requirements.md` DR-034 status corrected; parity matrix updated (crossfade ❌/❌, EQ row added).
+- [ ] Stale tauri#10152 comment in `nativeAdapter.ts` corrected.
+- [ ] The four sibling specs exist and are linked from here.
+
+## Testing
+
+n/a — documentation only.
+
+## TRACES
+
+No new code. Requirement text changes only; DR-034's status line is the one
+substantive edit.
+
+## Notes for the implementer
+
+- The evidence above was gathered in July 2026. The compositing constraint has
+  been stable since 2021 (wry#284) and is maintainer-declared unfixable, so it is
+  unlikely to change soon — but if someone revisits this, tauri#6343 and wry#284
+  are the threads to re-read first.
+- A parallel Claude session may be active in this repo — `git diff` before
+  "repairing" unexpected changes.
diff --git a/docs/specs/playback-docs-corrections.md b/docs/specs/playback-docs-corrections.md
new file mode 100644
index 00000000..91c509e5
--- /dev/null
+++ b/docs/specs/playback-docs-corrections.md
@@ -0,0 +1,198 @@
+# Spec: Playback documentation corrections
+
+**Status:** Proposed
+**Requirements:** revises the status of DR-034; corrects the parity matrix in [requirements.md](../requirements.md)
+**UX spec:** n/a
+**Supersedes / revises:** implements the "Corrections to existing docs" section of [playback-backend-unification.md](playback-backend-unification.md)
+
+## Summary
+
+Fix four factual errors in the requirements doc and the player source comments,
+all found while investigating backend unification. Each claims something the code
+does not do. Small change, but they are actively misleading: two of them assert a
+feature is implemented when it is implemented nowhere, and one cites an upstream
+blocker that no longer exists.
+
+Documentation and comments only — no behaviour change.
+
+## Motivation
+
+These errors compound. DR-034 reads "Done (Linux only)", so a future session
+planning Android parity would reasonably assume crossfade exists on Linux and
+only needs porting — when in fact it is unimplemented everywhere *and*
+architecturally blocked on the engine it supposedly runs on. Likewise the
+tauri#10152 comment has been discouraging work on Android native video since the
+upstream capability shipped in September 2024.
+
+## The corrections
+
+### 1. DR-034 status is wrong
+
+`requirements.md` line ~196:
+
+```
+| DR-034 | Crossfade engine with configurable duration (0-12s) | Player | UR-031 | Done (Linux only) |
+```
+
+The code:
+
+```rust
+// src-tauri/src/player/mpv_backend.rs, in set_audio_settings
+// TODO: Implement crossfade via MPV audio filters if needed
+```
+
+That is the entire crossfade implementation. `crossfade_duration` is plumbed
+through `AudioSettings` and clamped to 0–12s, but no backend ever acts on it.
+
+**Change to:** `Not implemented (blocked on MPV — see playback-backend-unification.md)`
+
+Worth stating *why* in the requirements entry, because it is not a scheduling
+gap: mpv's audio chain is single-stream, and FFmpeg's `acrossfade` is an `N→A`
+filter needing two inputs. Real crossfade requires two libmpv instances with
+manually ramped volumes. Upstream declined the feature (mpv#4512).
+
+### 1b. UR-031 status is wrong for the same reason
+
+`requirements.md` line ~44:
+
+```
+| UR-031 | Crossfade between audio tracks | Low | Done (Linux only) |
+```
+
+Same error one level up: the *user* requirement is also marked done. Since no
+backend implements crossfade, UR-031 is not satisfied on any platform.
+
+**Change to:** `Not implemented (blocked — see DR-034)`
+
+Note line ~517 of the same file (`UR-031 (Crossfade), UR-032 (Gapless),
+UR-033 (Normalization) only work on Linux`) inherits the error — crossfade works
+nowhere, so it should read UR-032/UR-033 only.
+
+### 2. Parity matrix crossfade row is wrong
+
+```
+| Crossfade | ✅ | ❌ | Gap |
+```
+
+**Change to** `| Crossfade | ❌ | ❌ | Not implemented |` — it is not a
+platform-parity gap, it is an unbuilt feature.
+
+### 3. Parity matrix is missing the equalizer
+
+The matrix lists crossfade, gapless, and normalization but omits the EQ, which
+has the same Linux-only shape and the same root cause (`ExoPlayerBackend` not
+overriding `set_audio_settings`). `build_af_filter` and `eq_filter_entries` exist
+only in `mpv_backend.rs`; there is no equalizer code in the Android tree.
+
+**Add:** `| Equalizer (10-band) | ✅ | ❌ | Gap |`
+
+### 4. `nativeAdapter.ts` cites a stale blocker
+
+`src/lib/player/adapters/nativeAdapter.ts:11-14` states native Android video is
+blocked upstream by tauri#10152 (transparent webview / SurfaceView compositing).
+
+tauri#10152 is open but **dead since 2024-07-01**, and it is a *feature request*
+that `WebviewWindowBuilder::transparent` was desktop-only — not a report that
+compositing is broken. The capability shipped in tauri commit `27d01834`
+(2024-09-02), which moved `transparent()` into the cross-platform impl block with
+only the tao call `#[cfg(desktop)]`-fenced. It landed as a clippy cleanup, so the
+issue was never closed. Separately, the black/white-screen bug (tauri#8381,
+tauri#9408) was a broken JNI signature for `setBackgroundColor`, fixed in wry
+0.39.4; we ship wry 0.55.x.
+
+**Change to:** a comment stating the adapter is currently unreachable because
+`createAdapter` hardcodes the HTML5 kind, that transparency is no longer an
+upstream blocker, and that
+[android-native-video-spike.md](android-native-video-spike.md) tracks whether
+SurfaceView compositing actually works. Be explicit that *nobody has
+demonstrated* SurfaceView-behind-WebView on Tauri Android — nothing upstream
+blocks it, and nothing upstream proves it.
+
+### 5. Platform capability is signalled three incompatible ways
+
+Not a doc error — a real inconsistency found during the same investigation, worth
+recording here even though fixing it needs its own change.
+
+Which backend a platform uses is currently expressed three ways:
+
+1. Rust `#[cfg]` gates in `player/mod.rs` and `create_player_backend` — the truth.
+2. The `useHtml5Element` / `VideoBackend` value from `get_player_status` — which
+   the frontend discards (see the spike spec).
+3. **Frontend user-agent sniffing** in `src/lib/services/webviewAudio.ts:30-41`:
+
+```ts
+const ua = navigator.userAgent.toLowerCase();
+const isAndroid = ua.includes("android");
+const isLinux = ua.includes("linux") && !isAndroid;
+return !isAndroid && !isLinux;
+```
+
+The comment says it is "matching the Rust cfg gate" — i.e. the frontend
+re-derives a backend decision from the user-agent string and hopes it stays in
+sync. That is the frontend deciding *which backend exists*, which is domain
+knowledge, not presentation. It also breaks silently the moment a new target is
+added or a webview's UA changes.
+
+**This is a boundary leak of the same family the spec-review checklist exists to
+catch**, even though `check:boundary`'s tripwire (item-type arrays) does not
+match it. Rust already computes the answer; the frontend should consume it.
+
+Not fixed by this spec — it is behavioural, not documentation. It should be
+folded into the spike spec's factory rework, where the same
+"consume Rust's decision instead of re-deriving it" change is already in scope.
+
+### Also worth fixing while here
+
+`requirements.md` IR-004 reads "In Progress (basic playback works, audio settings
+missing)". That stays accurate until
+[android-audio-settings-parity.md](android-audio-settings-parity.md) lands, but
+the "Future Fix" list in the parity issue proposes
+`ConcatenatingMediaSource` for crossfade — **deprecated in current Media3**. Drop
+that suggestion; the modern approach is a custom `AudioProcessor`.
+
+## Layer assignment
+
+No logic. Documentation and comments only.
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| — | — | No logic introduced or moved by this spec. |
+
+## Out of scope
+
+- Implementing crossfade. This spec only stops claiming it exists.
+- Implementing Android audio settings — see the parity spec.
+- Running the Android video spike — see that spec.
+- Rewriting the architecture docs. `docs/architecture/05-platform-backends.md`
+  should be re-read for the same class of error, but that is a larger pass.
+
+## Acceptance criteria
+
+- [ ] DR-034 status corrected, with the blocking reason stated.
+- [ ] UR-031 status corrected (line ~44), and the "only work on Linux" line (~517) no longer lists crossfade.
+- [ ] Parity matrix: crossfade ❌/❌; equalizer row added.
+- [ ] `ConcatenatingMediaSource` suggestion removed from the "Future Fix" list.
+- [ ] `nativeAdapter.ts` comment corrected and pointing at the spike spec.
+- [ ] `bun run check` and `bun run test` pass (a comment change still touches TS).
+- [ ] `bun run traces:markdown` re-run if requirement text changed.
+
+No Rust changes, so the `cargo` gates do not apply.
+
+## Testing
+
+None beyond the standard gates — no behaviour changes. Confirm
+`bun run traces:markdown` regenerates cleanly, since DR-034's row is referenced
+by the traceability matrix.
+
+## TRACES
+
+No code implementing requirements changes; no TRACES comments to add or update.
+The DR-034 row in `docs/traceability.md` will regenerate with the corrected text.
+
+## Notes for the implementer
+
+- Do **not** silently delete DR-034. The requirement (UR-031 crossfade) is still
+  wanted; it is the *status* that is wrong. Keeping the row with an honest status
+  and a reason is the point.
+- A parallel Claude session may be active — `git diff` before "repairing"
+  unexpected changes.
diff --git a/docs/specs/windows-native-audio-backend.md b/docs/specs/windows-native-audio-backend.md
new file mode 100644
index 00000000..920f7602
--- /dev/null
+++ b/docs/specs/windows-native-audio-backend.md
@@ -0,0 +1,209 @@
+# Spec: Windows native audio backend
+
+**Status:** Proposed
+**Requirements:** UR-003, UR-027, UR-032, UR-033 → DR-030, DR-035, DR-036; new IR-030
+**UX spec:** n/a — Settings › Audio already renders the controls
+**Supersedes / revises:** acts on the "audio can unify, video cannot" conclusion in [playback-backend-unification.md](playback-backend-unification.md)
+
+## Summary
+
+Give Windows a real native audio backend instead of the current webview
+`<audio>` shim. Windows is the only platform where audio playback has no decoder
+of its own: `WebviewAudioBackend` hands a URL to a frontend `<audio>` element and
+relays transport commands. It cannot set volume, cannot apply any audio setting,
+and reports state only via DOM events.
+
+Audio needs no rendering surface, so **none of the webview-compositing problems
+that block unified video apply here.** This is the cleanest available win.
+
+## Motivation
+
+`WebviewAudioBackend` was a deliberate stopgap ("audio-only playback for
+platforms without a native audio backend"), and it works — but it has a hard
+functional gap. From `webview_audio_backend.rs`:
+
+```rust
+fn set_volume(&mut self, volume: f32) -> Result<(), PlayerError> {
+    // ...stores locally only; there is no ControlCommand action for volume
+}
+```
+
+So volume changes never reach the element; the frontend has to observe the player
+store and apply volume itself. `set_audio_settings` likewise stores values that
+nothing consumes — EQ, normalization, and gapless are all inert on Windows.
+
+Meanwhile the backend-unification investigation established that a native *audio*
+engine is unproblematic on Windows specifically: `tauri-plugin-libmpv` lists
+Windows as its **fully tested** platform (in contrast to Linux, where embedding
+is broken — but that is a *video surface* problem, which audio does not have).
+
+## Layer assignment
+
+| Logic / responsibility | Layer | Why it belongs there |
+|------------------------|-------|----------------------|
+| Decoding and playing the audio stream | Rust | Playback is domain logic; every other platform already decodes in Rust or a native player. The webview shim is the anomaly. |
+| Applying `AudioSettings` (EQ/normalize/gapless) | Rust | Same `AudioSettings` contract as MPV/ExoPlayer; band layout and presets stay canonical in `settings.rs`. |
+| Position/state reporting | Rust | Restores the project's core principle — the player is the authoritative source of state. Today Windows inverts this: the DOM element is authoritative and Rust mirrors it. |
+| Volume | Rust | Currently broken precisely because it is split across the boundary. |
+| Rendering the player UI | Frontend | Unchanged. |
+
+The strongest argument for this change is the third row. CLAUDE.md states
+playback state is one-directional with the player authoritative; on Windows that
+is currently false, and the `player_report_*` round-trip exists to paper over it.
+
+## Design
+
+### Engine choice
+
+Two viable options; **libmpv is recommended** for consistency with the Linux
+audio backend.
+
+| | libmpv | GStreamer |
+|---|---|---|
+| Windows status | ✅ `tauri-plugin-libmpv` reports fully tested | ✅ works, but… |
+| Rust bindings | `libmpv2` 6.0.0, active | `gstreamer-rs` 0.25.x, excellent |
+| Cross-MSVC from Linux | ⚠️ needs prebuilt DLL + import lib | ❌ `gstreamer-sys` uses pkg-config, fights `cargo-xwin` |
+| Code reuse | ✅ `MpvBackend` logic is directly reusable | ❌ a second engine to learn |
+| Crossfade capable | ❌ single-stream chain | ✅ `audiomixer` |
+
+libmpv wins on reuse: `MpvBackend`'s `set_audio_settings` — the `af` lavfi graph
+built by `build_af_filter`, `eq_filter_entries`, `normalize_filter_entry` — is
+platform-independent and would apply unchanged.
+
+The one reason to prefer GStreamer is crossfade (UR-031), which mpv structurally
+cannot do. If crossfade becomes a priority, revisit; it would then argue for
+GStreamer on *both* Linux and Windows, which is a much larger change.
+
+### Structure
+
+Rename the cfg gate so `MpvBackend` is no longer Linux-only:
+
+```rust
+// src-tauri/src/player/mod.rs
+#[cfg(any(target_os = "linux", target_os = "windows"))]
+pub mod mpv_backend;
+```
+
+`MpvBackend::new` needs one platform-specific branch: `detect_audio_system()`
+currently probes `pactl`/`pw-cli`/`/proc/asound/cards` to pick an `ao`. On
+Windows the equivalent is `wasapi` (mpv's default), so the detection is a
+`#[cfg]` returning `"wasapi"` — no probing needed.
+
+Everything else — the event loop, the 250ms position thread, the seek-suppression
+window, the `af` filter graph — is unchanged.
+
+`WebviewAudioBackend` stays for other targets (macOS and anything else hitting
+the `not(any(...))` arm) and as the fallback if libmpv fails to initialize. The
+existing `emit_backend_init_failed` path already handles that gracefully.
+
+### Build
+
+`libmpv2-sys` is well-suited to cross-compilation: no pkg-config, vendored
+headers, pregenerated bindings (no libclang). It emits `cargo:rustc-link-lib=mpv`
+unconditionally, so the build must supply a linkable import library for
+`x86_64-pc-windows-msvc`.
+
+Keep the `build_libmpv` feature **off** — its Unix path shells out to mpv-build
+and explicitly rejects cross-compilation.
+
+🔴 Per CLAUDE.md, the prebuilt libmpv **must be added to the builder image**
+(`Dockerfile.builder` → rebuild + push via `scripts/build-builder-image.sh`), not
+installed at CI job time. `libmpv-2.dll` must also be bundled into the NSIS
+installer via `tauri.conf.json`'s resources.
+
+### Verified build mechanics
+
+The cross-compile path was tested hands-on from Linux (July 2026), not inferred:
+
+- Neither shinchiro nor zhongfly ships an `mpv.def` or MSVC `mpv.lib` — only a
+  MinGW `libmpv.dll.a`. (Several online sources claim otherwise; they are wrong.)
+- An MSVC-style import lib can be generated locally with LLVM tools only:
+  `llvm-readobj --coff-exports libmpv-2.dll` → synthesize `mpv.def` →
+  `llvm-dlltool -m i386:x86-64 -d mpv.def -l mpv.lib`. `llvm-lib /def:` produces a
+  byte-identical result.
+- A real `lld-link` link against that import lib **succeeds**, and the resulting
+  import table resolves `mpv_client_api_version` from `libmpv-2.dll`. `lld-link`
+  is the linker `cargo-xwin` uses, so this is the load-bearing step.
+- Linking directly against the shipped MinGW `libmpv.dll.a` **also** succeeds, so
+  def-generation may be skippable — but that relies on lld's GNU-archive
+  tolerance rather than a documented contract. Keep `llvm-dlltool` as the
+  fallback.
+- MinGW origin is not an ABI problem: libmpv exports a pure C ABI, and the x86-64
+  Windows calling convention is platform-defined. The upstream note that MSVC
+  cannot *build* mpv is frequently misread as "MSVC cannot *link* libmpv" — that
+  is not what it says.
+- 🔴 Never free/realloc across the DLL boundary — use `mpv_free`.
+
+Build wiring is ordinary: `cargo:rustc-link-lib=dylib=mpv` plus
+`cargo:rustc-link-search`. Nothing about libmpv conflicts with `cargo-xwin`.
+
+### Size and shipping
+
+Measured uncompressed: **93 MiB** (zhongfly `mpv-dev-lgpl-x86_64`) vs **112 MiB**
+(shinchiro, full GPL build); ~26–30 MB compressed in the `.7z`.
+
+**Ship the zhongfly LGPL build** — smaller, and there is no reason to pull the
+GPL variant in for an audio-only use.
+
+Import-table inspection confirms **no companion DLLs are needed**: every
+dependency is a system DLL (`KERNEL32`, `USER32`, `d2d1`, `DWrite`, `OPENGL32`,
+`vulkan-1`, UCRT `api-ms-win-*`). One file to bundle.
+
+93 MiB is still substantial against a Tauri app's usual few MB. Since we use mpv
+audio-only, investigate whether a pruned build (no video decoders, no libplacebo)
+is worth producing for the builder image — but treat that as an optimization,
+not a blocker.
+
+## Out of scope
+
+- Windows *video*. Stays in WebView2 + hls.js — it works and has ABR.
+- Crossfade (UR-031/DR-034) — not implemented anywhere; needs its own spec.
+- Replacing `WebviewAudioBackend` for macOS.
+- MPRIS/SMTC media-key integration — worth a follow-up, not this spec.
+
+## Acceptance criteria
+
+- [ ] Windows build produces a `MpvBackend`-backed player; `backend-init-failed` is emitted (not a crash) if libmpv is unavailable.
+- [ ] Volume control works from the UI — the current hard gap.
+- [ ] EQ, normalization, and gapless audibly take effect on Windows.
+- [ ] Position/state originate in Rust; the `<audio>` element is no longer in the audio path.
+- [ ] Seek, next/previous, and queue advance work; sleep timer stops playback.
+- [ ] `libmpv-2.dll` ships in the NSIS installer and the app runs on a clean Windows VM with no mpv installed.
+- [ ] Builder image carries the Windows libmpv artefacts; **no toolchain install added to any CI step**.
+- [ ] `bun run check`, `bun run test`, `bun run check:boundary` pass.
+- [ ] `cargo fmt` clean, `cargo clippy` clean, `bun run test:rust` passes.
+- [ ] New requirement-implementing code carries `// TRACES:` comments.
+
+## Testing
+
+**Rust**: the existing `mpv_backend_test.rs` and the `build_af_filter` /
+`normalize_filter_entry` / `eq_filter_entries` unit tests already cover the
+filter-graph logic and are platform-independent — they should pass unchanged
+under a Windows `cargo check`/test. Add a test asserting `detect_audio_system()`
+returns `wasapi` under `cfg(windows)`.
+
+**Manual, on Windows**: volume, EQ preset change, normalization toggle, gapless
+between two tracks, seek, queue advance, sleep timer. Then the packaging test —
+install the NSIS output on a clean VM and confirm it launches and plays.
+
+Per CLAUDE.md, the volume gap is a *bug fix*: write a failing test for
+"`set_volume` reaches the backend" before implementing.
+
+## TRACES
+
+- Windows `MpvBackend` construction in `create_player_backend` → `// TRACES: UR-003 | IR-030`
+- `detect_audio_system` Windows branch → `IR-030`
+- Existing `set_audio_settings` gains Windows coverage → `UR-027, UR-032, UR-033 | DR-030, DR-035, DR-036`
+- Allocate **IR-030** in `requirements.md` ("libmpv integration for Windows audio playback").
+
+## Notes for the implementer
+
+- Do this **after** [libmpv2-migration.md](libmpv2-migration.md) — porting the
+  current dead `libmpv` git pin to a second platform would double the migration
+  work.
+- `libmpv2` has broken its API in every major release (4.0 removed command
+  helpers, 5.0 removed `mpv_node`, 6.0 changed `RenderContext` ownership). Pin an
+  exact version.
+- Only the `render`-feature parts of `libmpv2` concern video; audio-only use does
+  not need it, and disabling the default `render` feature may shrink the build.
+- A parallel Claude session may be active — `git diff` first.
diff --git a/src/lib/player/adapters/nativeAdapter.ts b/src/lib/player/adapters/nativeAdapter.ts
index e1102c00..bae252b1 100644
--- a/src/lib/player/adapters/nativeAdapter.ts
+++ b/src/lib/player/adapters/nativeAdapter.ts
@@ -7,11 +7,19 @@
  * hls.js. State reporting is unnecessary here because the native backend emits
  * events directly — the adapter's job is only to forward control intents.
  *
- * NOTE: On current Tauri, native Android video rendering is blocked upstream
- * (transparent webview / SurfaceView compositing — tauri#10152), so video on
- * Android currently runs through the HTML5 adapter via the interim override in
- * the factory. This adapter exists for the audio/native path and for when that
- * upstream limitation is resolved.
+ * NOTE: This adapter is currently unreachable — `createAdapter()` hardcodes the
+ * HTML5 kind, so Android video runs through Html5PlayerAdapter.
+ *
+ * That override was introduced citing tauri#10152 as an upstream blocker. That
+ * is no longer accurate: #10152 is a stale *feature request* (dead since
+ * 2024-07-01) asking that `transparent` not be desktop-only, and the capability
+ * shipped in tauri commit 27d01834 (2024-09-02). The related black/white-screen
+ * bug (tauri#8381, #9408) was a broken JNI signature for setBackgroundColor,
+ * fixed in wry 0.39.4; we ship wry 0.55.x.
+ *
+ * What is genuinely unproven is SurfaceView-behind-WebView *compositing* on
+ * Tauri Android — nothing upstream blocks it, and nothing upstream demonstrates
+ * it either. docs/specs/android-native-video-spike.md tracks that experiment.
  *
  * TRACES: UR-003, UR-005 | DR-004, DR-028
  */