[codex] Prefer unlabeled external sidecars for local playback (#46)

Co-authored-by: bee <autumn@skerritt.blog>

backlog/tasks/task-279 - Fix-Linux-AppImage-child-process-libffmpeg-resolution.md

@@ -0,0 +1,62 @@
+---
+id: TASK-279
+title: Fix Linux AppImage child-process libffmpeg resolution
+status: Done
+assignee:
+  - '@codex'
+created_date: '2026-04-05 17:17'
+updated_date: '2026-04-05 17:56'
+labels: []
+dependencies: []
+references:
+  - 'https://github.com/ksyasuda/SubMiner/issues/41'
+documentation:
+  - docs/workflow/verification.md
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Fix the Linux AppImage packaging so Chromium child processes relaunched from the bundled binary can resolve the packaged libffmpeg shared library and SubMiner starts cleanly instead of crash-looping on network-service restarts.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Linux AppImage packaging ensures bundled Chromium child processes can resolve the packaged libffmpeg shared library during relaunch.
+- [x] #2 Regression coverage exercises the Linux packaging/build configuration that provides the AppImage shared-library path.
+- [x] #3 Release notes/changelog reflect the Linux AppImage startup fix.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add focused regression tests for Linux release packaging that assert the build config invokes an `afterPack` hook and that the hook stages bundled `libffmpeg.so` into `usr/lib` for AppImage runtime lookup.
+2. Implement a small electron-builder `afterPack` hook that runs only for Linux, copies `libffmpeg.so` from the packaged app root into `usr/lib`, and no-ops when the source library is absent.
+3. Wire the hook into `package.json` build config and add a changelog fragment for the Linux AppImage startup fix.
+4. Run the focused test lane first, then the default handoff gate because the change touches release-sensitive packaging behavior.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Chose a repo-local electron-builder `afterPack` hook instead of patching/forking `electron-builder`. The hook copies bundled `libffmpeg.so` from the packaged Linux app root into `usr/lib`, matching the AppImage runtime's existing `LD_LIBRARY_PATH` search path.
+
+Added regression coverage for both config wiring (`src/release-workflow.test.ts`) and the hook behavior (`scripts/electron-builder-after-pack.test.ts`), then wired the new script test into `test:fast` so the maintained lane keeps exercising the fix.
+
+Verification passed: `bun test scripts/electron-builder-after-pack.test.ts src/release-workflow.test.ts`, `bun run typecheck`, `bun run test:fast`, `bun run test:env`, `bun run build`, `bun run test:smoke:dist`.
+
+Addressed PR #45 CodeRabbit review thread: Linux `afterPack` staging now hard-fails when `libffmpeg.so` is missing instead of silently no-oping. Updated focused hook tests to assert the new failure contract and that `afterPack` propagates Linux staging errors.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Added a shared electron-builder `afterPack` hook at `scripts/electron-builder-after-pack.cjs` and wired it into `package.json` so Linux packaging stages the bundled `libffmpeg.so` into `usr/lib` inside the packaged app. This keeps Chromium child relaunches compatible with the AppImage runtime's existing `LD_LIBRARY_PATH` layout without forking or patching upstream `electron-builder`.
+
+Regression coverage now checks both the packaging config and the hook behavior: `src/release-workflow.test.ts` asserts the hook stays wired into release config, and `scripts/electron-builder-after-pack.test.ts` verifies Linux copies `libffmpeg.so` into `usr/lib` while non-Linux and missing-library cases no-op safely. The new script test is included in `test:fast`, and a changelog fragment was added under `changes/fix-appimage-libffmpeg-path.md`.
+
+Verification passed with `bun test scripts/electron-builder-after-pack.test.ts src/release-workflow.test.ts`, `bun run typecheck`, `bun run test:fast`, `bun run test:env`, `bun run build`, and `bun run test:smoke:dist`.
+
+Follow-up review fix on PR #45: Linux packaging now throws when `libffmpeg.so` is missing from the packaged app root, preventing silent shipment of a broken AppImage. Focused regression coverage was updated so the missing-library case rejects and `afterPack` propagates the failure.
+<!-- SECTION:FINAL_SUMMARY:END -->

backlog/tasks/task-280 - Force-launcher-spawned-mpv-onto-X11-when-backend-resolves-to-x11-or-no-supported-Wayland-tracker-is-available.md

@@ -0,0 +1,59 @@
+---
+id: TASK-280
+title: >-
+  Force launcher-spawned mpv onto X11 when backend resolves to x11 or no
+  supported Wayland tracker is available
+status: Done
+assignee:
+  - codex
+created_date: '2026-04-05 21:01'
+updated_date: '2026-04-05 21:05'
+labels:
+  - bug
+  - linux
+  - launcher
+  - overlay
+dependencies: []
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+On Linux Plasma Wayland and similar sessions, `subminer --backend=x11` currently only changes SubMiner's window-tracker override. The launcher still spawns mpv without forcing an X11/XWayland backend, so the X11 tracker cannot find the mpv window and the overlay remains hidden. Update launcher-side mpv spawn behavior so launcher-managed mpv runs under X11 when backend resolves to `x11`, and also when auto detection cannot resolve to a supported Wayland tracker. Preserve existing Hyprland/Sway behavior.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Launcher-managed mpv is spawned with X11/XWayland-forcing environment/config when backend resolves to `x11`.
+- [x] #2 Linux auto mode falls back to X11/XWayland-forced mpv when no supported Wayland tracker backend is detected.
+- [x] #3 Hyprland and Sway launcher flows do not regress to forced X11 mpv.
+- [x] #4 Regression tests cover launcher env/backend selection for these Linux cases.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add focused launcher tests that assert the mpv spawn environment forces X11 when backend resolves to `x11`, and when Linux auto mode cannot use a supported Wayland tracker.
+2. Refactor launcher mpv spawn code to compute an mpv-specific environment without changing existing Hyprland/Sway flows.
+3. Route all launcher-managed mpv spawns through the new environment helper.
+4. Run focused launcher tests, then summarize behavior and any remaining verification gaps.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+User approved scope: force launcher-managed mpv to X11 for explicit `--backend=x11` and for unsupported Linux Wayland auto-detect fallback; preserve Hyprland/Sway behavior.
+
+Implemented launcher-side `buildMpvEnv` to strip Wayland hints and force X11/XWayland for launcher-managed mpv when `--backend=x11`, and for Linux auto mode on unsupported Wayland desktops with an X11 display available. Wired both normal mpv launches and idle detached mpv launches through the helper.
+
+Verification: `bun test launcher/mpv.test.ts --test-name-pattern "buildMpvEnv"` passed; `bun run tsc --noEmit` passed. A broader `bun test launcher/mpv.test.ts` run still hits a pre-existing sandbox-specific failure in `launchAppCommandDetached handles child process spawn errors` because this environment cannot write the default app log path under `/home/sudacode/.config/SubMiner/logs`.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Updated the launcher so mpv gets an X11/XWayland-oriented spawn environment whenever the user explicitly requests `--backend=x11`, and when Linux auto mode is running under an unsupported Wayland desktop that still exposes an X11 display. The new helper reuses the launcher child-process base environment, strips Wayland-specific hints (`WAYLAND_DISPLAY`, Hyprland/Sway markers), and flips `XDG_SESSION_TYPE` to `x11` only for those fallback cases. Both foreground mpv launches and detached idle mpv launches now use the same helper so overlay-tracked playback stays consistent.
+
+Added focused regression coverage in `launcher/mpv.test.ts` for three cases: explicit `x11` forcing, unsupported Wayland auto fallback (for example KDE Plasma Wayland), and preserving native Wayland env for supported Hyprland/Sway auto backends. Verification completed with `bun test launcher/mpv.test.ts --test-name-pattern "buildMpvEnv"` and `bun run tsc --noEmit`. A broader launcher mpv test run still shows an unrelated sandbox write failure for the default app log path in this environment.
+<!-- SECTION:FINAL_SUMMARY:END -->

backlog/tasks/task-281 - Prevent-Windows-launcher-tests-from-leaking-backslash-temp-files-on-POSIX.md

@@ -0,0 +1,62 @@
+---
+id: TASK-281
+title: Prevent Windows launcher tests from leaking backslash temp files on POSIX
+status: Done
+assignee:
+  - codex
+created_date: '2026-04-05 21:13'
+updated_date: '2026-04-05 21:20'
+labels:
+  - tests
+  - launcher
+  - bug
+dependencies: []
+documentation:
+  - /home/sudacode/github/SubMiner2/AGENTS.md
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Windows-specific launcher tests in launcher/mpv.test.ts currently create real filesystem entries using path.win32.join(...) with a POSIX mkdtemp base. On Linux/macOS this produces literal backslash-named paths like \\tmp\\subminer-test-win-dir-* inside the repo/worktree, and the existing cleanup only removes the POSIX /tmp base directory. Fix the tests so they still cover Windows path resolution behavior without leaking stray files into the working tree.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Running the Windows findAppBinary tests on a POSIX host does not create new untracked \\tmp\\subminer-test-win-* files in the repository root.
+- [x] #2 The Windows launcher tests still validate PATH and install-directory resolution behavior.
+- [x] #3 Relevant launcher tests pass after the change.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add a regression test in launcher/mpv.test.ts that exercises the Windows findAppBinary cases on a POSIX host and asserts they do not leave new backslash-named temp artifacts in the repository root.
+2. Refactor the Windows launcher tests to avoid creating real filesystem paths from path.win32.join(...) on POSIX; keep Windows path assertions via stubs and only create real files with native POSIX paths where needed.
+3. Run the targeted launcher tests and confirm no new \\tmp\\subminer-test-win-* artifacts appear in git status.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Investigation: reproduced the leak locally. The source is launcher/mpv.test.ts Windows findAppBinary tests that combine a POSIX mkdtemp base with path.win32.join(...), creating literal backslash-named entries like \\tmp\\subminer-test-win-dir-* in the repo root. Existing cleanup only removes the POSIX /tmp base directory.
+
+User approved implementation plan on 2026-04-05.
+
+Implemented in launcher/mpv.test.ts by replacing the leaky Windows PATH/install-directory helpers with pure fs stubs (access/exists/stat) and fixed Windows path strings instead of creating real path.win32 filesystem entries on POSIX. Added a regression test that snapshots repo-root \\tmp\\subminer-test-win-* artifacts before/after running the Windows cases and asserts no new entries are created.
+
+Verification: `bun test launcher/mpv.test.ts --test-name-pattern 'findAppBinary Windows cases do not leak backslash temp artifacts on POSIX|findAppBinary resolves SubMiner.exe on PATH on Windows|findAppBinary resolves a Windows install directory to SubMiner.exe'` passed (3/3). `bun test launcher/mpv.test.ts` still has one unrelated pre-existing sandbox failure in `launchAppCommandDetached handles child process spawn errors` because the test opens `~/.config/SubMiner/logs/app-2026-04-05.log` and hits `EROFS` in this environment.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Reworked the Windows `findAppBinary` tests in `launcher/mpv.test.ts` so they no longer create real backslash-named temp files on POSIX hosts. The PATH and install-directory cases now use synthetic Windows path strings plus `fs.accessSync` / `fs.existsSync` / `fs.statSync` stubs to exercise the same resolver behavior without writing `\\tmp\\subminer-test-win-*` entries into the repository root.
+
+Added a POSIX regression test that snapshots existing repo-root `\\tmp\\subminer-test-win-*` artifacts, runs the Windows path-resolution cases, and asserts the artifact set is unchanged. This catches future regressions where a Windows-path test accidentally writes literal backslash paths on Linux/macOS.
+
+Tests run:
+- `bun test launcher/mpv.test.ts --test-name-pattern 'findAppBinary Windows cases do not leak backslash temp artifacts on POSIX|findAppBinary resolves SubMiner.exe on PATH on Windows|findAppBinary resolves a Windows install directory to SubMiner.exe'`
+- `bun test launcher/mpv.test.ts` (all relevant `findAppBinary` tests passed; one unrelated existing sandbox failure remains in `launchAppCommandDetached handles child process spawn errors` due `EROFS` opening `~/.config/SubMiner/logs/...`)
+<!-- SECTION:FINAL_SUMMARY:END -->

backlog/tasks/task-282 - Force-launcher-managed-mpv-to-an-explicit-X11-GPU-context-when-X11-fallback-is-active.md

@@ -0,0 +1,58 @@
+---
+id: TASK-282
+title: >-
+  Force launcher-managed mpv to an explicit X11 GPU context when X11 fallback is
+  active
+status: Done
+assignee:
+  - codex
+created_date: '2026-04-05 21:14'
+updated_date: '2026-04-05 21:15'
+labels:
+  - bug
+  - linux
+  - launcher
+  - mpv
+  - overlay
+dependencies: []
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Follow-up to the launcher X11 fallback work: on Plasma Wayland, stripping Wayland env vars alone is not sufficient for launcher-managed mpv. mpv can still fail GPU initialization under the default video output path (`vo/gpu-next`) unless an explicit X11 GPU context is selected. Update launcher-managed mpv startup so X11 fallback mode also appends explicit mpv options for an X11 GPU context, while preserving supported Hyprland/Sway Wayland flows.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Launcher-managed mpv appends explicit X11 GPU context args when explicit `--backend=x11` is used on Linux.
+- [x] #2 Launcher-managed mpv appends the same explicit X11 GPU context args for Linux auto-mode fallback on unsupported Wayland desktops.
+- [x] #3 Supported Hyprland/Sway Wayland flows do not receive the forced X11 GPU context args.
+- [x] #4 Regression tests cover the forced-X11 mpv arg selection.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add focused launcher tests for explicit X11 GPU-context arg selection, unsupported Wayland auto fallback, and Hyprland/Sway no-regression cases.
+2. Introduce a shared launcher helper that decides when mpv should be forced onto X11 fallback mode.
+3. Use that helper to append explicit mpv X11 GPU-context args in both normal and detached idle mpv launch paths.
+4. Run focused launcher tests plus TypeScript verification, then record remaining runtime follow-up guidance.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+User runtime log showed `vo/gpu-next` failing with `Failed initializing any suitable GPU context!` under forced-X11 playback, which indicates env forcing alone was insufficient. Selected `--gpu-context=x11egl,x11` as the explicit mpv fallback: prefer X11/EGL, with GLX as a compatibility fallback.
+
+Verification: `bun test launcher/mpv.test.ts --test-name-pattern "buildMpv(Env|BackendArgs)"` passed. `bun run tsc --noEmit` passed.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Added explicit mpv backend args for launcher-managed X11 fallback mode. The launcher now uses a shared `shouldForceX11MpvBackend` decision for both env rewriting and mpv arg selection, so explicit `--backend=x11` and unsupported Linux Wayland auto fallback both append `--gpu-context=x11egl,x11` while still stripping Wayland env hints. This preserves supported Hyprland/Sway native Wayland flows and makes the X11 fallback more explicit for mpv's GPU initialization path.
+
+Wired the new X11 GPU-context args into both the normal playback launch path and the detached idle mpv launch path. Added focused regression coverage for explicit `x11`, Plasma-style unsupported Wayland auto fallback, and Hyprland/Sway no-regression behavior. Verification completed with `bun test launcher/mpv.test.ts --test-name-pattern "buildMpv(Env|BackendArgs)"` and `bun run tsc --noEmit`.
+<!-- SECTION:FINAL_SUMMARY:END -->

backlog/tasks/task-283 - Force-launcher-managed-X11-fallback-to-mpv-vogpu-with-OpenGL-on-Linux.md

@@ -0,0 +1,56 @@
+---
+id: TASK-283
+title: Force launcher-managed X11 fallback to mpv vo=gpu with OpenGL on Linux
+status: Done
+assignee:
+  - codex
+created_date: '2026-04-05 21:19'
+updated_date: '2026-04-05 21:20'
+labels:
+  - bug
+  - linux
+  - launcher
+  - mpv
+  - overlay
+dependencies: []
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Follow-up to explicit X11 gpu-context forcing: Plasma Wayland runtime logs still show mpv using `vo/gpu-next` and failing to initialize any suitable GPU context under launcher-managed X11 fallback. Update launcher-managed mpv X11 fallback mode to force a more compatible renderer stack: `--vo=gpu`, `--gpu-api=opengl`, and `--gpu-context=x11egl,x11`, while preserving supported native Wayland flows and allowing explicit user mpv args to override later on the command line.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Launcher-managed mpv appends `--vo=gpu`, `--gpu-api=opengl`, and `--gpu-context=x11egl,x11` when explicit `--backend=x11` is used on Linux.
+- [x] #2 Launcher-managed mpv appends the same renderer args for Linux auto-mode fallback on unsupported Wayland desktops.
+- [x] #3 Supported Hyprland/Sway Wayland flows do not receive the forced X11 renderer args.
+- [x] #4 Regression tests cover the forced-X11 renderer arg selection.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Tighten launcher tests so forced-X11 renderer args require `--vo=gpu`, `--gpu-api=opengl`, and `--gpu-context=x11egl,x11`.
+2. Update the shared launcher X11-fallback helper to return the full renderer arg stack for explicit `x11` and unsupported Wayland auto fallback.
+3. Re-run focused launcher env/backend tests and TypeScript verification.
+4. Hand back with retry instructions and next debugging branch if runtime still fails.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Runtime log still showed `[vo/gpu-next] Failed initializing any suitable GPU context!`, which meant forcing only the context was not enough. Updated the fallback to force the classic OpenGL renderer path too: `--vo=gpu --gpu-api=opengl --gpu-context=x11egl,x11`.
+
+Verification: `bun test launcher/mpv.test.ts --test-name-pattern "buildMpv(Env|BackendArgs)"` passed. `bun run tsc --noEmit` passed.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Updated launcher-managed X11 fallback mode to force a more compatible mpv renderer stack on Linux: `--vo=gpu`, `--gpu-api=opengl`, and `--gpu-context=x11egl,x11`. This applies both to explicit `--backend=x11` and to unsupported Wayland auto fallback, while supported Hyprland/Sway Wayland sessions still keep their native path. The renderer args are still inserted before user-supplied `--args`, so an explicit user override can win later on the command line if needed.
+
+Adjusted regression coverage to require the full renderer stack for forced-X11 mode and verified the helper behavior with focused launcher tests plus TypeScript compilation. Verification completed with `bun test launcher/mpv.test.ts --test-name-pattern "buildMpv(Env|BackendArgs)"` and `bun run tsc --noEmit`.
+<!-- SECTION:FINAL_SUMMARY:END -->

backlog/tasks/task-284 - Fix-CI-changelog-fragment-requirement-for-launcher-X11-MPV-fallback-change.md

@@ -0,0 +1,32 @@
+---
+id: TASK-284
+title: Fix CI changelog fragment requirement for launcher X11 MPV fallback change
+status: Done
+assignee: []
+created_date: '2026-04-05 22:21'
+updated_date: '2026-04-05 22:22'
+labels:
+  - ci
+  - changelog
+dependencies: []
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Current CI is failing in `changelog:pr-check` because PR changes release-relevant files but does not include a required entry under `changes/` and lacks `skip-changelog` label. Add a release fragment describing the behavioral change and verify the CI gate passes.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Add a correctly formatted changelog fragment under `changes/` for the current change
+- [x] #2 Run the local changelog PR check (or equivalent) with passing result
+- [x] #3 Run required CI gate commands after change and confirm no regressions
+<!-- AC:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Added `changes/2026.04.05-mpv-x11-fallback.md` with launcher release-note metadata so `changelog:pr-check` can pass. Verified local CI gate commands: `bun run typecheck`, `bun run test:fast`, `bun run test:env`, `bun run build`, `bun run test:smoke:dist` all passed. Ran manual PR changelog verification by invoking `verifyPullRequestChangelog` with current git diff plus the new fragment and confirmed it passes.
+<!-- SECTION:FINAL_SUMMARY:END -->

changes/2026.04.05-mpv-x11-fallback.md

@@ -0,0 +1,4 @@
+type: fixed
+area: launcher
+
+- Fixed launcher-managed mpv spawning to force an explicit X11 GPU path when Wayland trackers are unavailable.

changes/43-local-subtitle-sidecar.md

@@ -0,0 +1,4 @@
+type: fixed
+area: launcher
+
+Local playback now promotes a single unlabeled external subtitle sidecar to the primary slot instead of leaving mpv's embedded English auto-selection in place.

changes/fix-appimage-libffmpeg-path.md

@@ -0,0 +1,4 @@
+type: fixed
+area: release
+
+- Fixed Linux AppImage startup packaging so Chromium child relaunches can resolve the bundled `libffmpeg.so` instead of crash-looping on startup.

launcher/mpv.test.ts

@@ -7,6 +7,8 @@ import net from 'node:net';
 import { EventEmitter } from 'node:events';
 import type { Args } from './types';
 import {
+  buildMpvBackendArgs,
+  buildMpvEnv,
   cleanupPlaybackSession,
   detectBackend,
   findAppBinary,
@@ -125,6 +127,113 @@ test('detectBackend resolves windows on win32 auto mode', () => {
   });
 });
 
+test('buildMpvEnv forces X11 by dropping Wayland hints when backend resolves to x11', () => {
+  withPlatform('linux', () => {
+    const env = buildMpvEnv(makeArgs({ backend: 'x11' }), {
+      DISPLAY: ':1',
+      WAYLAND_DISPLAY: 'wayland-0',
+      XDG_SESSION_TYPE: 'wayland',
+      HYPRLAND_INSTANCE_SIGNATURE: 'hypr',
+      SWAYSOCK: '/tmp/sway.sock',
+    });
+
+    assert.equal(env.DISPLAY, ':1');
+    assert.equal(env.WAYLAND_DISPLAY, undefined);
+    assert.equal(env.XDG_SESSION_TYPE, 'x11');
+    assert.equal(env.HYPRLAND_INSTANCE_SIGNATURE, undefined);
+    assert.equal(env.SWAYSOCK, undefined);
+  });
+});
+
+test('buildMpvEnv auto mode falls back to X11 when no supported Wayland tracker backend is detected', () => {
+  withPlatform('linux', () => {
+    const env = buildMpvEnv(makeArgs({ backend: 'auto' }), {
+      DISPLAY: ':1',
+      WAYLAND_DISPLAY: 'wayland-0',
+      XDG_SESSION_TYPE: 'wayland',
+      XDG_CURRENT_DESKTOP: 'KDE',
+      XDG_SESSION_DESKTOP: 'plasma',
+    });
+
+    assert.equal(env.DISPLAY, ':1');
+    assert.equal(env.WAYLAND_DISPLAY, undefined);
+    assert.equal(env.XDG_SESSION_TYPE, 'x11');
+  });
+});
+
+test('buildMpvEnv preserves native Wayland env for supported Hyprland and Sway auto backends', () => {
+  withPlatform('linux', () => {
+    const hyprEnv = buildMpvEnv(makeArgs({ backend: 'auto' }), {
+      DISPLAY: ':1',
+      WAYLAND_DISPLAY: 'wayland-0',
+      XDG_SESSION_TYPE: 'wayland',
+      HYPRLAND_INSTANCE_SIGNATURE: 'hypr',
+    });
+    assert.equal(hyprEnv.WAYLAND_DISPLAY, 'wayland-0');
+    assert.equal(hyprEnv.XDG_SESSION_TYPE, 'wayland');
+
+    const swayEnv = buildMpvEnv(makeArgs({ backend: 'auto' }), {
+      DISPLAY: ':1',
+      WAYLAND_DISPLAY: 'wayland-0',
+      XDG_SESSION_TYPE: 'wayland',
+      SWAYSOCK: '/tmp/sway.sock',
+    });
+    assert.equal(swayEnv.WAYLAND_DISPLAY, 'wayland-0');
+    assert.equal(swayEnv.XDG_SESSION_TYPE, 'wayland');
+  });
+});
+
+test('buildMpvBackendArgs forces an explicit X11 renderer stack when backend resolves to x11', () => {
+  withPlatform('linux', () => {
+    assert.deepEqual(
+      buildMpvBackendArgs(makeArgs({ backend: 'x11' }), {
+        DISPLAY: ':1',
+        WAYLAND_DISPLAY: 'wayland-0',
+        XDG_SESSION_TYPE: 'wayland',
+      }),
+      ['--vo=gpu', '--gpu-api=opengl', '--gpu-context=x11egl,x11'],
+    );
+  });
+});
+
+test('buildMpvBackendArgs forces the same X11 renderer stack for unsupported Wayland auto fallback', () => {
+  withPlatform('linux', () => {
+    assert.deepEqual(
+      buildMpvBackendArgs(makeArgs({ backend: 'auto' }), {
+        DISPLAY: ':1',
+        WAYLAND_DISPLAY: 'wayland-0',
+        XDG_SESSION_TYPE: 'wayland',
+        XDG_CURRENT_DESKTOP: 'KDE',
+        XDG_SESSION_DESKTOP: 'plasma',
+      }),
+      ['--vo=gpu', '--gpu-api=opengl', '--gpu-context=x11egl,x11'],
+    );
+  });
+});
+
+test('buildMpvBackendArgs keeps supported Hyprland and Sway auto backends unchanged', () => {
+  withPlatform('linux', () => {
+    assert.deepEqual(
+      buildMpvBackendArgs(makeArgs({ backend: 'auto' }), {
+        DISPLAY: ':1',
+        WAYLAND_DISPLAY: 'wayland-0',
+        XDG_SESSION_TYPE: 'wayland',
+        HYPRLAND_INSTANCE_SIGNATURE: 'hypr',
+      }),
+      [],
+    );
+    assert.deepEqual(
+      buildMpvBackendArgs(makeArgs({ backend: 'auto' }), {
+        DISPLAY: ':1',
+        WAYLAND_DISPLAY: 'wayland-0',
+        XDG_SESSION_TYPE: 'wayland',
+        SWAYSOCK: '/tmp/sway.sock',
+      }),
+      [],
+    );
+  });
+});
+
 test('launchTexthookerOnly exits non-zero when app binary cannot be spawned', () => {
   const error = withProcessExitIntercept(() => {
     launchTexthookerOnly('/definitely-missing-subminer-binary', makeArgs());
@@ -485,6 +594,40 @@ function withAccessSyncStub(
   }
 }
 
+function withExistsAndStatSyncStubs(
+  options: {
+    existingPaths?: string[];
+    directoryPaths?: string[];
+  },
+  run: () => void,
+): void {
+  const existingPaths = new Set(options.existingPaths ?? []);
+  const directoryPaths = new Set(options.directoryPaths ?? []);
+  const originalExistsSync = fs.existsSync;
+  const originalStatSync = fs.statSync;
+  try {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (fs as any).existsSync = (filePath: string): boolean =>
+      existingPaths.has(filePath) || directoryPaths.has(filePath);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (fs as any).statSync = (filePath: string) => {
+      if (directoryPaths.has(filePath)) {
+        return { isDirectory: () => true };
+      }
+      if (existingPaths.has(filePath)) {
+        return { isDirectory: () => false };
+      }
+      throw Object.assign(new Error(`ENOENT: ${filePath}`), { code: 'ENOENT' });
+    };
+    run();
+  } finally {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (fs as any).existsSync = originalExistsSync;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (fs as any).statSync = originalStatSync;
+  }
+}
+
 function withRealpathSyncStub(resolvePath: (filePath: string) => string, run: () => void): void {
   const originalRealpathSync = fs.realpathSync;
   try {
@@ -497,6 +640,75 @@ function withRealpathSyncStub(resolvePath: (filePath: string) => string, run: ()
   }
 }
 
+function listRepoRootWindowsTempArtifacts(): string[] {
+  return fs
+    .readdirSync(process.cwd())
+    .filter((entry) => /^\\tmp\\subminer-test-win-/.test(entry))
+    .sort();
+}
+
+function runFindAppBinaryWindowsPathCase(): void {
+  const baseDir = 'C:\\Users\\tester\\subminer-test-win-path';
+  const originalHomedir = os.homedir;
+  const originalPath = process.env.PATH;
+  try {
+    os.homedir = () => baseDir;
+    const binDir = path.win32.join(baseDir, 'bin');
+    const wrapperPath = path.win32.join(binDir, 'SubMiner.exe');
+    process.env.PATH = `${binDir}${path.win32.delimiter}${originalPath ?? ''}`;
+
+    withFindAppBinaryPlatformSandbox('win32', (pathModule) => {
+      withAccessSyncStub(
+        (filePath) => filePath === wrapperPath,
+        () => {
+          const result = findAppBinary(
+            pathModule.join(baseDir, 'launcher', 'SubMiner.exe'),
+            pathModule,
+          );
+          assert.equal(result, wrapperPath);
+        },
+      );
+    });
+  } finally {
+    os.homedir = originalHomedir;
+    process.env.PATH = originalPath;
+  }
+}
+
+function runFindAppBinaryWindowsInstallDirCase(): void {
+  const baseDir = 'C:\\Users\\tester\\subminer-test-win-dir';
+  const originalHomedir = os.homedir;
+  const originalSubminerBinaryPath = process.env.SUBMINER_BINARY_PATH;
+  try {
+    os.homedir = () => baseDir;
+    const installDir = path.win32.join(baseDir, 'Programs', 'SubMiner');
+    const appExe = path.win32.join(installDir, 'SubMiner.exe');
+    process.env.SUBMINER_BINARY_PATH = installDir;
+
+    withPlatform('win32', () => {
+      withExistsAndStatSyncStubs(
+        { existingPaths: [appExe], directoryPaths: [installDir] },
+        () => {
+          withAccessSyncStub(
+            (filePath) => filePath === appExe,
+            () => {
+              const result = findAppBinary(path.win32.join(baseDir, 'launcher', 'SubMiner.exe'), path.win32);
+              assert.equal(result, appExe);
+            },
+          );
+        },
+      );
+    });
+  } finally {
+    os.homedir = originalHomedir;
+    if (originalSubminerBinaryPath === undefined) {
+      delete process.env.SUBMINER_BINARY_PATH;
+    } else {
+      process.env.SUBMINER_BINARY_PATH = originalSubminerBinaryPath;
+    }
+  }
+}
+
 test(
   'findAppBinary resolves ~/.local/bin/SubMiner.AppImage when it exists',
   { concurrency: false },
@@ -657,71 +869,31 @@ test('findAppBinary resolves Windows install paths when present', { concurrency:
   }
 });
 
-test('findAppBinary resolves SubMiner.exe on PATH on Windows', { concurrency: false }, () => {
-  const baseDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-test-win-path-'));
-  const originalHomedir = os.homedir;
-  const originalPath = process.env.PATH;
-  try {
-    os.homedir = () => baseDir;
-    const binDir = path.win32.join(baseDir, 'bin');
-    const wrapperPath = path.win32.join(binDir, 'SubMiner.exe');
-    makeExecutable(wrapperPath);
-    process.env.PATH = `${binDir}${path.win32.delimiter}${originalPath ?? ''}`;
-
-    withFindAppBinaryPlatformSandbox('win32', (pathModule) => {
-      withAccessSyncStub(
-        (filePath) => filePath === wrapperPath,
+test(
+  'findAppBinary Windows cases do not leak backslash temp artifacts on POSIX',
+  { concurrency: false },
   () => {
-          const result = findAppBinary(
-            pathModule.join(baseDir, 'launcher', 'SubMiner.exe'),
-            pathModule,
-          );
-          assert.equal(result, wrapperPath);
+    if (path.sep === '\\') {
+      return;
+    }
+
+    const before = listRepoRootWindowsTempArtifacts();
+    runFindAppBinaryWindowsPathCase();
+    runFindAppBinaryWindowsInstallDirCase();
+    const after = listRepoRootWindowsTempArtifacts();
+
+    assert.deepEqual(after, before);
   },
 );
-    });
-  } finally {
-    os.homedir = originalHomedir;
-    process.env.PATH = originalPath;
-    fs.rmSync(baseDir, { recursive: true, force: true });
-  }
+
+test('findAppBinary resolves SubMiner.exe on PATH on Windows', { concurrency: false }, () => {
+  runFindAppBinaryWindowsPathCase();
 });
 
 test(
   'findAppBinary resolves a Windows install directory to SubMiner.exe',
   { concurrency: false },
   () => {
-    const baseDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-test-win-dir-'));
-    const originalHomedir = os.homedir;
-    const originalSubminerBinaryPath = process.env.SUBMINER_BINARY_PATH;
-    try {
-      os.homedir = () => baseDir;
-      const installDir = path.win32.join(baseDir, 'Programs', 'SubMiner');
-      const appExe = path.win32.join(installDir, 'SubMiner.exe');
-      process.env.SUBMINER_BINARY_PATH = installDir;
-      fs.mkdirSync(installDir, { recursive: true });
-      fs.writeFileSync(appExe, '#!/bin/sh\nexit 0\n');
-      fs.chmodSync(appExe, 0o755);
-
-      const originalPlatform = process.platform;
-      try {
-        Object.defineProperty(process, 'platform', { value: 'win32', configurable: true });
-        const result = findAppBinary(
-          path.win32.join(baseDir, 'launcher', 'SubMiner.exe'),
-          path.win32,
-        );
-        assert.equal(result, appExe);
-      } finally {
-        Object.defineProperty(process, 'platform', { value: originalPlatform, configurable: true });
-      }
-    } finally {
-      os.homedir = originalHomedir;
-      if (originalSubminerBinaryPath === undefined) {
-        delete process.env.SUBMINER_BINARY_PATH;
-      } else {
-        process.env.SUBMINER_BINARY_PATH = originalSubminerBinaryPath;
-      }
-      fs.rmSync(baseDir, { recursive: true, force: true });
-    }
+    runFindAppBinaryWindowsInstallDirCase();
   },
 );

launcher/mpv.ts

@@ -225,27 +225,65 @@ export function makeTempDir(prefix: string): string {
   return fs.mkdtempSync(path.join(os.tmpdir(), prefix));
 }
 
-export function detectBackend(backend: Backend): Exclude<Backend, 'auto'> {
+export function detectBackend(
+  backend: Backend,
+  env: NodeJS.ProcessEnv = process.env,
+): Exclude<Backend, 'auto'> {
   if (backend !== 'auto') return backend;
   if (process.platform === 'win32') return 'windows';
   if (process.platform === 'darwin') return 'macos';
-  const xdgCurrentDesktop = (process.env.XDG_CURRENT_DESKTOP || '').toLowerCase();
-  const xdgSessionDesktop = (process.env.XDG_SESSION_DESKTOP || '').toLowerCase();
-  const xdgSessionType = (process.env.XDG_SESSION_TYPE || '').toLowerCase();
-  const hasWayland = Boolean(process.env.WAYLAND_DISPLAY) || xdgSessionType === 'wayland';
+  const linuxDesktopEnv = getLinuxDesktopEnv(env);
 
   if (
-    process.env.HYPRLAND_INSTANCE_SIGNATURE ||
-    xdgCurrentDesktop.includes('hyprland') ||
-    xdgSessionDesktop.includes('hyprland')
+    env.HYPRLAND_INSTANCE_SIGNATURE ||
+    linuxDesktopEnv.xdgCurrentDesktop.includes('hyprland') ||
+    linuxDesktopEnv.xdgSessionDesktop.includes('hyprland')
   ) {
     return 'hyprland';
   }
-  if (hasWayland && commandExists('hyprctl')) return 'hyprland';
-  if (process.env.DISPLAY) return 'x11';
+  if (linuxDesktopEnv.hasWayland && commandExists('hyprctl')) return 'hyprland';
+  if (env.DISPLAY) return 'x11';
   fail('Could not detect display backend');
 }
 
+type LinuxDesktopEnv = {
+  xdgCurrentDesktop: string;
+  xdgSessionDesktop: string;
+  hasWayland: boolean;
+};
+
+function getLinuxDesktopEnv(env: NodeJS.ProcessEnv): LinuxDesktopEnv {
+  const xdgCurrentDesktop = (env.XDG_CURRENT_DESKTOP || '').toLowerCase();
+  const xdgSessionDesktop = (env.XDG_SESSION_DESKTOP || '').toLowerCase();
+  const xdgSessionType = (env.XDG_SESSION_TYPE || '').toLowerCase();
+  return {
+    xdgCurrentDesktop,
+    xdgSessionDesktop,
+    hasWayland: Boolean(env.WAYLAND_DISPLAY) || xdgSessionType === 'wayland',
+  };
+}
+
+function shouldForceX11MpvBackend(
+  args: Pick<Args, 'backend'>,
+  env: NodeJS.ProcessEnv,
+): boolean {
+  if (process.platform !== 'linux' || !env.DISPLAY?.trim()) {
+    return false;
+  }
+
+  const linuxDesktopEnv = getLinuxDesktopEnv(env);
+  const supportedWaylandBackend =
+    Boolean(env.HYPRLAND_INSTANCE_SIGNATURE || env.SWAYSOCK) ||
+    linuxDesktopEnv.xdgCurrentDesktop.includes('hyprland') ||
+    linuxDesktopEnv.xdgCurrentDesktop.includes('sway') ||
+    linuxDesktopEnv.xdgSessionDesktop.includes('hyprland') ||
+    linuxDesktopEnv.xdgSessionDesktop.includes('sway');
+  return (
+    args.backend === 'x11' ||
+    (args.backend === 'auto' && linuxDesktopEnv.hasWayland && !supportedWaylandBackend)
+  );
+}
+
 function resolveAppBinaryCandidate(candidate: string, pathModule: PathModule = path): string {
   const direct = resolveBinaryPathCandidate(candidate);
   if (!direct) return '';
@@ -637,6 +675,7 @@ export async function startMpv(
   const mpvArgs: string[] = [];
   if (args.profile) mpvArgs.push(`--profile=${args.profile}`);
   mpvArgs.push(...DEFAULT_MPV_SUBMINER_ARGS);
+  mpvArgs.push(...buildMpvBackendArgs(args));
   if (targetKind === 'url' && isYoutubeTarget(target)) {
     log('info', args.logLevel, 'Applying URL playback options');
     mpvArgs.push('--ytdl=yes');
@@ -712,7 +751,10 @@ export async function startMpv(
   const mpvTarget = resolveCommandInvocation('mpv', mpvArgs, {
     normalizeWindowsShellArgs: false,
   });
-  state.mpvProc = spawn(mpvTarget.command, mpvTarget.args, { stdio: 'inherit' });
+  state.mpvProc = spawn(mpvTarget.command, mpvTarget.args, {
+    stdio: 'inherit',
+    env: buildMpvEnv(args),
+  });
 }
 
 async function waitForOverlayStartCommandSettled(
@@ -889,9 +931,9 @@ function stopManagedOverlayApp(args: Args): void {
   }
 }
 
-function buildAppEnv(): NodeJS.ProcessEnv {
+function buildAppEnv(baseEnv: NodeJS.ProcessEnv = process.env): NodeJS.ProcessEnv {
   const env: Record<string, string | undefined> = {
-    ...process.env,
+    ...baseEnv,
     SUBMINER_APP_LOG: getAppLogPath(),
     SUBMINER_MPV_LOG: getMpvLogPath(),
   };
@@ -911,6 +953,32 @@ function buildAppEnv(): NodeJS.ProcessEnv {
   return env;
 }
 
+export function buildMpvEnv(
+  args: Pick<Args, 'backend'>,
+  baseEnv: NodeJS.ProcessEnv = process.env,
+): NodeJS.ProcessEnv {
+  const env = buildAppEnv(baseEnv);
+  if (!shouldForceX11MpvBackend(args, env)) {
+    return env;
+  }
+
+  delete env.WAYLAND_DISPLAY;
+  delete env.HYPRLAND_INSTANCE_SIGNATURE;
+  delete env.SWAYSOCK;
+  env.XDG_SESSION_TYPE = 'x11';
+  return env;
+}
+
+export function buildMpvBackendArgs(
+  args: Pick<Args, 'backend'>,
+  baseEnv: NodeJS.ProcessEnv = process.env,
+): string[] {
+  if (!shouldForceX11MpvBackend(args, baseEnv)) {
+    return [];
+  }
+  return ['--vo=gpu', '--gpu-api=opengl', '--gpu-context=x11egl,x11'];
+}
+
 function appendCapturedAppOutput(kind: 'STDOUT' | 'STDERR', chunk: string): void {
   const normalized = chunk.replace(/\r\n/g, '\n');
   for (const line of normalized.split('\n')) {
@@ -1144,6 +1212,7 @@ export function launchMpvIdleDetached(
     const mpvArgs: string[] = [];
     if (args.profile) mpvArgs.push(`--profile=${args.profile}`);
     mpvArgs.push(...DEFAULT_MPV_SUBMINER_ARGS);
+    mpvArgs.push(...buildMpvBackendArgs(args));
     if (args.mpvArgs) {
       mpvArgs.push(...parseMpvArgString(args.mpvArgs));
     }
@@ -1159,6 +1228,7 @@ export function launchMpvIdleDetached(
     const proc = spawn(mpvTarget.command, mpvTarget.args, {
       stdio: 'ignore',
       detached: true,
+      env: buildMpvEnv(args),
     });
     if (typeof proc.pid === 'number' && proc.pid > 0) {
       trackDetachedMpvPid(proc.pid);

package.json

@@ -68,7 +68,7 @@
     "test:launcher": "bun run test:launcher:src",
     "test:core": "bun run test:core:src",
     "test:subtitle": "bun run test:subtitle:src",
-    "test:fast": "bun run test:config:src && bun run test:core:src && bun run test:docs:kb && bun test src/main-entry-runtime.test.ts src/anki-integration.test.ts src/anki-integration/anki-connect-proxy.test.ts src/anki-integration/field-grouping-workflow.test.ts src/anki-integration/field-grouping.test.ts src/anki-integration/field-grouping-merge.test.ts src/release-workflow.test.ts src/ci-workflow.test.ts scripts/build-changelog.test.ts scripts/mkv-to-readme-video.test.ts scripts/run-coverage-lane.test.ts scripts/update-aur-package.test.ts && bun test src/core/services/immersion-tracker/__tests__/query.test.ts src/core/services/immersion-tracker/__tests__/query-split-modules.test.ts && bun run tsc && bun test dist/main/runtime/registry.test.js",
+    "test:fast": "bun run test:config:src && bun run test:core:src && bun run test:docs:kb && bun test src/main-entry-runtime.test.ts src/anki-integration.test.ts src/anki-integration/anki-connect-proxy.test.ts src/anki-integration/field-grouping-workflow.test.ts src/anki-integration/field-grouping.test.ts src/anki-integration/field-grouping-merge.test.ts src/release-workflow.test.ts src/ci-workflow.test.ts scripts/build-changelog.test.ts scripts/electron-builder-after-pack.test.ts scripts/mkv-to-readme-video.test.ts scripts/run-coverage-lane.test.ts scripts/update-aur-package.test.ts && bun test src/core/services/immersion-tracker/__tests__/query.test.ts src/core/services/immersion-tracker/__tests__/query-split-modules.test.ts && bun run tsc && bun test dist/main/runtime/registry.test.js",
     "generate:config-example": "bun run src/generate-config-example.ts",
     "verify:config-example": "bun run src/verify-config-example.ts",
     "start": "bun run build && electron . --start",
@@ -128,6 +128,7 @@
     "productName": "SubMiner",
     "executableName": "SubMiner",
     "artifactName": "SubMiner-${version}.${ext}",
+    "afterPack": "scripts/electron-builder-after-pack.cjs",
     "icon": "assets/SubMiner-square.png",
     "directories": {
       "output": "release"

scripts/electron-builder-after-pack.cjs

@@ -0,0 +1,46 @@
+const fs = require('node:fs/promises');
+const path = require('node:path');
+
+const LINUX_FFMPEG_LIBRARY = 'libffmpeg.so';
+
+async function stageLinuxAppImageSharedLibrary(
+  context,
+  deps = {
+    access: (filePath) => fs.access(filePath),
+    mkdir: (dirPath) => fs.mkdir(dirPath, { recursive: true }),
+    copyFile: (from, to) => fs.copyFile(from, to),
+  },
+) {
+  if (context.electronPlatformName !== 'linux') return false;
+
+  const sourceLibraryPath = path.join(context.appOutDir, LINUX_FFMPEG_LIBRARY);
+
+  try {
+    await deps.access(sourceLibraryPath);
+  } catch (error) {
+    if (error && typeof error === 'object' && error.code === 'ENOENT') {
+      throw new Error(
+        `Linux packaging requires ${LINUX_FFMPEG_LIBRARY} at ${sourceLibraryPath} so AppImage child processes can resolve it.`,
+      );
+    }
+    throw error;
+  }
+
+  const targetLibraryDir = path.join(context.appOutDir, 'usr', 'lib');
+  const targetLibraryPath = path.join(targetLibraryDir, LINUX_FFMPEG_LIBRARY);
+
+  await deps.mkdir(targetLibraryDir);
+  await deps.copyFile(sourceLibraryPath, targetLibraryPath);
+
+  return true;
+}
+
+async function afterPack(context) {
+  await stageLinuxAppImageSharedLibrary(context);
+}
+
+module.exports = {
+  LINUX_FFMPEG_LIBRARY,
+  stageLinuxAppImageSharedLibrary,
+  default: afterPack,
+};

scripts/electron-builder-after-pack.test.ts

@@ -0,0 +1,104 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+
+const {
+  LINUX_FFMPEG_LIBRARY,
+  default: afterPack,
+  stageLinuxAppImageSharedLibrary,
+} = require('./electron-builder-after-pack.cjs') as {
+  LINUX_FFMPEG_LIBRARY: string;
+  default: (context: { appOutDir: string; electronPlatformName: string }) => Promise<void>;
+  stageLinuxAppImageSharedLibrary: (context: {
+    appOutDir: string;
+    electronPlatformName: string;
+  }) => Promise<boolean>;
+};
+
+function createWorkspace(name: string): string {
+  return fs.mkdtempSync(path.join(os.tmpdir(), `${name}-`));
+}
+
+test('stageLinuxAppImageSharedLibrary copies libffmpeg.so into usr/lib for Linux packaging', async () => {
+  const workspace = createWorkspace('subminer-after-pack-linux');
+  const appOutDir = path.join(workspace, 'SubMiner-linux-x64');
+  const sourceLibraryPath = path.join(appOutDir, LINUX_FFMPEG_LIBRARY);
+  const targetLibraryPath = path.join(appOutDir, 'usr', 'lib', LINUX_FFMPEG_LIBRARY);
+
+  fs.mkdirSync(appOutDir, { recursive: true });
+  fs.writeFileSync(sourceLibraryPath, 'bundled ffmpeg', 'utf8');
+
+  try {
+    const staged = await stageLinuxAppImageSharedLibrary({
+      appOutDir,
+      electronPlatformName: 'linux',
+    });
+
+    assert.equal(staged, true);
+    assert.equal(fs.readFileSync(targetLibraryPath, 'utf8'), 'bundled ffmpeg');
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});
+
+test('stageLinuxAppImageSharedLibrary skips non-Linux packaging contexts', async () => {
+  const workspace = createWorkspace('subminer-after-pack-non-linux');
+  const appOutDir = path.join(workspace, 'SubMiner-darwin-arm64');
+  const sourceLibraryPath = path.join(appOutDir, LINUX_FFMPEG_LIBRARY);
+  const targetLibraryPath = path.join(appOutDir, 'usr', 'lib', LINUX_FFMPEG_LIBRARY);
+
+  fs.mkdirSync(appOutDir, { recursive: true });
+  fs.writeFileSync(sourceLibraryPath, 'bundled ffmpeg', 'utf8');
+
+  try {
+    const staged = await stageLinuxAppImageSharedLibrary({
+      appOutDir,
+      electronPlatformName: 'darwin',
+    });
+
+    assert.equal(staged, false);
+    assert.equal(fs.existsSync(targetLibraryPath), false);
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});
+
+test('stageLinuxAppImageSharedLibrary throws when Linux packaging is missing libffmpeg.so', async () => {
+  const workspace = createWorkspace('subminer-after-pack-missing-library');
+  const appOutDir = path.join(workspace, 'SubMiner-linux-x64');
+
+  fs.mkdirSync(appOutDir, { recursive: true });
+
+  try {
+    await assert.rejects(
+      stageLinuxAppImageSharedLibrary({
+        appOutDir,
+        electronPlatformName: 'linux',
+      }),
+      new RegExp(`Linux packaging requires ${LINUX_FFMPEG_LIBRARY} at .*${LINUX_FFMPEG_LIBRARY}`),
+    );
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});
+
+test('afterPack propagates Linux staging failures', async () => {
+  const workspace = createWorkspace('subminer-after-pack-propagates-linux-failure');
+  const appOutDir = path.join(workspace, 'SubMiner-linux-x64');
+
+  fs.mkdirSync(appOutDir, { recursive: true });
+
+  try {
+    await assert.rejects(
+      afterPack({
+        appOutDir,
+        electronPlatformName: 'linux',
+      }),
+      /Linux packaging requires libffmpeg\.so/,
+    );
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});

src/main/runtime/local-subtitle-selection.test.ts

@@ -15,6 +15,11 @@ const mixedLanguageTrackList = [
   { type: 'sub', id: 12, lang: 'ja', title: 'ja.srt', external: true },
 ];
 
+const unlabeledExternalSidecarTrackList = [
+  { type: 'sub', id: 1, lang: 'eng', title: 'English ASS', external: false, selected: true },
+  { type: 'sub', id: 2, title: 'srt', external: true },
+];
+
 test('resolveManagedLocalSubtitleSelection prefers default Japanese primary and English secondary tracks', () => {
   const result = resolveManagedLocalSubtitleSelection({
     trackList: mixedLanguageTrackList,
@@ -37,6 +42,31 @@ test('resolveManagedLocalSubtitleSelection respects configured language override
   assert.equal(result.secondaryTrackId, 12);
 });
 
+test('resolveManagedLocalSubtitleSelection promotes a single unlabeled external sidecar to primary', () => {
+  const result = resolveManagedLocalSubtitleSelection({
+    trackList: unlabeledExternalSidecarTrackList,
+    primaryLanguages: [],
+    secondaryLanguages: [],
+  });
+
+  assert.equal(result.primaryTrackId, 2);
+  assert.equal(result.secondaryTrackId, 1);
+});
+
+test('resolveManagedLocalSubtitleSelection does not guess between multiple unlabeled external sidecars', () => {
+  const result = resolveManagedLocalSubtitleSelection({
+    trackList: [
+      ...unlabeledExternalSidecarTrackList,
+      { type: 'sub', id: 3, title: 'subrip', external: true },
+    ],
+    primaryLanguages: [],
+    secondaryLanguages: [],
+  });
+
+  assert.equal(result.primaryTrackId, null);
+  assert.equal(result.secondaryTrackId, 1);
+});
+
 test('managed local subtitle selection runtime applies preferred tracks once for a local media path', async () => {
   const commands: Array<Array<string | number>> = [];
   const scheduled: Array<() => void> = [];
@@ -75,3 +105,42 @@ test('managed local subtitle selection runtime applies preferred tracks once for
     ['set_property', 'secondary-sid', 11],
   ]);
 });
+
+test('managed local subtitle selection runtime promotes a single unlabeled external sidecar over embedded english', async () => {
+  const commands: Array<Array<string | number>> = [];
+  const scheduled: Array<() => void> = [];
+
+  const runtime = createManagedLocalSubtitleSelectionRuntime({
+    getCurrentMediaPath: () => '/videos/example.mkv',
+    getMpvClient: () =>
+      ({
+        connected: true,
+        requestProperty: async (name: string) => {
+          if (name === 'track-list') {
+            return unlabeledExternalSidecarTrackList;
+          }
+          throw new Error(`Unexpected property: ${name}`);
+        },
+      }) as never,
+    getPrimarySubtitleLanguages: () => [],
+    getSecondarySubtitleLanguages: () => [],
+    sendMpvCommand: (command) => {
+      commands.push(command);
+    },
+    schedule: (callback) => {
+      scheduled.push(callback);
+      return 1 as never;
+    },
+    clearScheduled: () => {},
+  });
+
+  runtime.handleMediaPathChange('/videos/example.mkv');
+  scheduled.shift()?.();
+  await new Promise((resolve) => setTimeout(resolve, 0));
+  runtime.handleSubtitleTrackListChange(unlabeledExternalSidecarTrackList);
+
+  assert.deepEqual(commands, [
+    ['set_property', 'sid', 2],
+    ['set_property', 'secondary-sid', 1],
+  ]);
+});

src/main/runtime/local-subtitle-selection.ts

@@ -90,6 +90,10 @@ function isLikelyHearingImpaired(title: string): boolean {
   return HEARING_IMPAIRED_PATTERN.test(title);
 }
 
+function isUnlabeledExternalTrack(track: NormalizedSubtitleTrack): boolean {
+  return track.external && normalizeYoutubeLangCode(track.lang).length === 0;
+}
+
 function pickBestTrackId(
   tracks: NormalizedSubtitleTrack[],
   preferredLanguages: string[],
@@ -126,6 +130,19 @@ function pickBestTrackId(
   };
 }
 
+function pickSingleUnlabeledExternalTrackId(
+  tracks: NormalizedSubtitleTrack[],
+  excludeId: number | null = null,
+): number | null {
+  const fallbackCandidates = tracks.filter(
+    (track) => track.id !== excludeId && isUnlabeledExternalTrack(track),
+  );
+  if (fallbackCandidates.length !== 1) {
+    return null;
+  }
+  return fallbackCandidates[0]?.id ?? null;
+}
+
 export function resolveManagedLocalSubtitleSelection(input: {
   trackList: unknown[] | null;
   primaryLanguages: string[];
@@ -146,12 +163,13 @@ export function resolveManagedLocalSubtitleSelection(input: {
   );
 
   const primary = pickBestTrackId(tracks, preferredPrimaryLanguages);
-  const secondary = pickBestTrackId(tracks, preferredSecondaryLanguages, primary.trackId);
+  const primaryTrackId = primary.trackId ?? pickSingleUnlabeledExternalTrackId(tracks);
+  const secondary = pickBestTrackId(tracks, preferredSecondaryLanguages, primaryTrackId);
 
   return {
-    primaryTrackId: primary.trackId,
+    primaryTrackId,
     secondaryTrackId: secondary.trackId,
-    hasPrimaryMatch: primary.hasMatch,
+    hasPrimaryMatch: primary.hasMatch || primaryTrackId !== null,
     hasSecondaryMatch: secondary.hasMatch,
   };
 }

src/release-workflow.test.ts

@@ -13,6 +13,7 @@ const packageJson = JSON.parse(readFileSync(packageJsonPath, 'utf8')) as {
   productName?: string;
   scripts: Record<string, string>;
   build?: {
+    afterPack?: string;
     files?: string[];
   };
 };
@@ -77,6 +78,10 @@ test('release package scripts disable implicit electron-builder publishing', ()
   assert.match(packageJson.scripts['build:win:unsigned'] ?? '', /build-win-unsigned\.mjs/);
 });
 
+test('release packaging wires a shared afterPack hook for Linux AppImage library staging', () => {
+  assert.equal(packageJson.build?.afterPack, 'scripts/electron-builder-after-pack.cjs');
+});
+
 test('top-level package metadata keeps Linux Electron runtime app identity canonical', () => {
   assert.equal(packageJson.productName, 'SubMiner');
   assert.equal(packageJson.desktopName, 'SubMiner.desktop');