chore(windows): add audio device-state watcher for headphone dropout diagnosis (#724)

[abduznik]

Fixes #724 — a wireless headset (Corsair Void Wireless V2, but this affects most wireless headsets) disconnects partway through a mic-only recording.

Honest framing: this is a workaround, not a root-cause fix

The disconnect is caused by the headset's own firmware idle-power-off timer — confirmed via a clean, controlled diff of every PnP/WASAPI device property (371 properties total) on the headset and its USB dongle, taken once with the headset off and once on: zero differences. Windows has no visibility into the wireless link state at any layer this process can observe, so there is nothing to detect or intercept from inside openscreen.

What this PR does instead is give the firmware what its own timer is already watching for: real audio activity on the render endpoint. It does not disable, detect, or otherwise touch the underlying timer — the mechanism stays exactly as the manufacturer built it. This works because most wireless headsets implement the same class of firmware behavior (JBL, Turtle Beach, Logitech, and Corsair all have documented versions of an inactivity auto-off), so a generic "keep real signal flowing to the render endpoint" fix applies broadly rather than needing per-vendor work.

We deliberately did not go the alternative route of reverse-engineering each headset's USB HID protocol to disable the timer directly (confirmed feasible for this exact Corsair chipset via Sapd/HeadsetControl's setInactiveTime(0)), because that path is vendor/model-specific indefinitely, needs an embedded HID library, and permanently changes the user's hardware setting rather than scoping the workaround to when openscreen actually needs the device awake.

Root cause detail

WasapiLoopbackCapture's system-audio path only ever reads from the render endpoint via AUDCLNT_STREAMFLAGS_LOOPBACK. A mic-only recording never touches the render endpoint at all. Testing on real hardware:

• Mic-only (system audio off): headset disconnects at ~7-10 minutes. Reproduced repeatedly.
• Mic + system audio (loopback capture reads real content from the render endpoint when audio is playing): headset stays connected.

A first attempt wrote AUDCLNT_BUFFERFLAGS_SILENT packets (matching "touch the endpoint" but not "real content") and did not reliably prevent the drop under controlled testing — it looked like it worked once, then failed at ~630-640s in a clean re-test with all other apps closed, landing right back in the original unfixed timing window. That ruled out "any activity, even silence" and pointed at needing genuine signal, consistent with the loopback-capture comparison above.

What changed

• WasapiRenderKeepAlive (electron/native/wgc-capture/src/wasapi_render_keepalive.{h,cpp}) — opens the default render endpoint in shared mode and writes a 19kHz tone at 0.3% amplitude (adaptively capped below Nyquist for unusually low sample rates) for the duration of the recording. 19kHz is above what the large majority of adults can hear; validated inaudible in testing. Only runs when system audio capture is off — loopback capture already keeps the endpoint busy with real content when it's on, and running both would additionally capture the tone into the recording's system-audio track. Shared mode so it can't block another app from using the device; any failure (no output device, another app holding it exclusively, etc.) is non-fatal to the recording. Kill-switch: OPENSCREEN_WGC_DISABLE_AUDIO_KEEPALIVE=1.
• WasapiDeviceWatcher (electron/native/wgc-capture/src/wasapi_device_watcher.{h,cpp}) — diagnostic-only IMMNotificationClient that logs render/capture endpoint state transitions as structured JSON events. This is what proved the drop is invisible at the WASAPI layer (never fired during a confirmed drop) and is kept in as a reusable diagnostic for future audio-hardware reports. Opt-in via OPENSCREEN_WGC_LOG_AUDIO_DEVICE_EVENTS=1. IMMNotificationClient callbacks are nonblocking per Microsoft's documented contract — they only copy their arguments and hand off to a dedicated worker thread that does name resolution and the actual write.
• Both wired into main.cpp via a single stop-helper lambda each, called at every exit path, logged as their own named [stop-timing] steps.

Testing

Standalone diagnostic tool (scripts/diagnostic-tool/diagnostic.mjs), on the reporter's real Corsair Void Wireless V2 hardware, clean conditions (all other apps closed) each time:

• No fix: mic-only, 600-700s — headset dropped. Reproduced multiple times.
• Silent keep-alive: mic-only, 600s — stayed connected once, then dropped at ~630-640s on a controlled re-test. Inconsistent, ruled out.
• 1kHz tone @ 1%: prevented the drop, but was clearly audible.
• 19kHz tone @ 0.3%: prevented the drop for the full 700s, confirmed inaudible.

CI: Windows x64 diagnostic bundle and Rust check (Windows compositor) pass, confirming clean compilation.

Still open

• A tone-based mic-continuity test (à la test-windows-audio-timeline.mjs) proving the mic track stays continuous and undamaged with the keep-alive running.
• Confirming the recorded system-audio track stays clean when system audio capture is on (keep-alive is a no-op in that case by design, but worth a regression test to lock that in).
• "Inaudible" is validated for the reporter's hearing and headset, not a hard guarantee for every listener or every driver's frequency response — worth a note in the PR/docs.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

Summary by CodeRabbit

• Bug Fixes

  • Improved microphone-only recording reliability by preventing compatible wireless headsets from idling or disconnecting.
  • Audio device monitoring now processes events without blocking capture operations.
  • Audio support shuts down cleanly across recording startup failures and stopping scenarios.
  • Audio keep-alive failures no longer prevent recordings from starting.

• Diagnostics

  • Added optional logging of audio render and capture device state changes during recordings.
  • Device events include endpoint details, state changes, direction, and timestamps.

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📝 Walkthrough

Walkthrough

The capture executable now builds and manages an optional WASAPI device watcher and render keep-alive stream. The watcher queues notifications for asynchronous JSON output. The keep-alive writes a near-inaudible tone during mic-only recordings. Both stop on capture failures and normal shutdown.

Changes

WASAPI capture support

Layer / File(s)	Summary
Render keep-alive implementation electron/native/wgc-capture/src/wasapi_render_keepalive.h, electron/native/wgc-capture/src/wasapi_render_keepalive.cpp	Adds a shared-mode render stream that writes a continuous near-inaudible 19 kHz tone. The helper validates the sample rate, manages COM initialization, renders supported PCM formats, and shuts down its worker and WASAPI resources.
Device notification implementation electron/native/wgc-capture/src/wasapi_device_watcher.h, electron/native/wgc-capture/src/wasapi_device_watcher.cpp	Queues notification data from IMMNotificationClient callbacks. A worker resolves device metadata and writes baseline and device-state JSON events.
Capture lifecycle integration electron/native/wgc-capture/CMakeLists.txt, electron/native/wgc-capture/src/main.cpp	Builds both helpers, enables the watcher through OPENSCREEN_WGC_LOG_AUDIO_DEVICE_EVENTS, enables the keep-alive for recordings without system-audio capture unless disabled, and stops active helpers on startup failures, timeout, and normal shutdown.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant main
  participant WasapiDeviceWatcher
  participant WasapiRenderKeepAlive
  participant WASAPIEndpoints
  main->>WasapiDeviceWatcher: start when device logging is enabled
  main->>WasapiRenderKeepAlive: start for mic-only capture unless disabled
  WasapiRenderKeepAlive->>WASAPIEndpoints: open render stream and write tone
  WASAPIEndpoints-->>WasapiDeviceWatcher: deliver device notifications
  main->>WasapiDeviceWatcher: stop on failure or shutdown
  main->>WasapiRenderKeepAlive: stop on failure or shutdown

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the Windows audio device-state watcher and its headphone dropout diagnosis purpose. It does not mention the render keep-alive mitigation, but it accurately describes a sig…
Description check	✅ Passed	The description is detailed and covers the change, related issue, implementation, testing, limitations, and follow-up work. It omits the template's Type of change, Release impact, and Desktop impact s…
Linked Issues check	✅ Passed	The pull request meets the coding objective in issue #724. WasapiRenderKeepAlive opens the shared default render endpoint and writes a continuous low-amplitude 19 kHz signal during microphone-only r…
Out of Scope Changes check	✅ Passed	The changes stay within issue #724. WasapiDeviceWatcher provides opt-in diagnostics for render and capture endpoint state changes during the same recording path. Its asynchronous callback handling, …

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Inline comments:
In `@electron/native/wgc-capture/src/wasapi_device_watcher.cpp`:
- Around line 153-164: Update WasapiDeviceWatcher::emitDeviceEvent so
IMMNotificationClient callbacks only copy the event type, deviceId, and
extraJson into owned data and enqueue it without waiting on outputMutex_,
resolving the friendly name, or flushing output. Add a worker to dequeue events,
resolve names, and write output; during shutdown, unregister callbacks, drain
the queue, join the worker, and only then reset deviceEnumerator_.
- Around line 156-164: Update WasapiDeviceWatcher::emitDeviceEvent and the JSON
event-writing flow in main.cpp to construct each complete event before output
and route all events through one shared synchronized emitter protected by the
common mutex. Ensure no direct chained writes bypass this emitter, preserving
one complete JSON record per line without interleaving.

In `@electron/native/wgc-capture/src/wasapi_render_keepalive.cpp`:
- Around line 79-80: Initialize COM within the lambda that starts renderLoop:
call CoInitializeEx(nullptr, COINIT_MULTITHREADED) before renderLoop(), call
CoUninitialize() after it returns only when initialization succeeds, and
preserve the existing thread join and interface-release ordering.

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Inline comments:
In `@electron/native/wgc-capture/src/wasapi_render_keepalive.cpp`:
- Line 73: Update writeToneFrames to zero-initialize the entire output buffer
for frameCount multiplied by format->nBlockAlign before generating samples,
ensuring unsupported wBitsPerSample formats render silence while preserving
existing sample generation behavior.
- Line 46: Update the tone-frequency calculation in start() so the keep-alive
tone remains above the audible range while respecting the Nyquist limit, using
the intended 19 kHz target. If the endpoint’s Nyquist frequency cannot provide
that target with the required margin, return false from start() before starting
the stream.

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