fix(linux): enumerate screens and windows in separate calls under X11

[EtienneLescot]

Summary

Cause found. On X11, a single desktopCapturer.getSources call that asks for screen and window together makes the X11 screen capturer's thumbnail grab fail now and then (Failed to initialize pixel buffer). What the caller sees depends on Electron:

• Electron 41: the call never settles. This is the hang CI: desktop source enumeration does not return on ~2 of 5 attempts on GitHub runners #462 describes: ~17 ms or never.
• Electron 43 (what the flake ships today): the call settles after Electron's 3 s ready timer with the screen dropped. The nix-build smoke now shows this as returned 0 source(s) in 3003ms, i.e. displays: [], 4 of 5 times in run 37134061785.

Fix.

• The get-sources handler asks for one type per call, one call after the other, on Linux X11 only. This covers openscreen sources, openscreen record and the GUI pickers.
• Wayland keeps the single call. There Electron serves both types from one PipeWire capturer, so splitting would open the portal twice.
• Windows and macOS keep the single call.
• nix-build.yml now counts an attempt that exits 0 with displays: [] as a failure. The verdict requires all 5 attempts, as the old comment asked once the cause was known.

Related issue

Refs #462

Testing

Reproduced under Xvfb in WSL with a minimal Electron app, using the flake's xvfb-run and its Electron 43.4.1 plus a stored Electron 41.0.2. Eight runs per cell:

request	Electron 41	Electron 43
screen,window in one call	4/8 hang	3/8 lose the screen
screen then window	0/8	0/8
window then screen	0/8	0/8
screen alone (18 calls each)	0	0

• Each failure came with exactly one Failed to initialize pixel buffer line, and runs without a failure had none.
• After a hung combined call, a screen-only call in the same process still answered in about 12 ms.

The packaged app, A/B. I ran nix build .#openscreen in WSL with a local npmDepsHash patch, then openscreen sources 10 times under xvfb-run -a, the way the smoke step does:

build	runs with a display
main	2/10 (the other 8 returned displays: [])
this branch	10/10

The record probe now finds the screen. It still fails after that, with Recording did not start within 30s. That is a separate capture problem under Xvfb, and the probe stays informational.

Unit tests and checks.

• electron/ipc/desktopSourceTypes.test.ts checks the platform and Wayland gating, one type per call, that calls never overlap, the pass-through cases and error propagation.
• tsc passes for the app and test configs. Biome is clean.

Not proven.

• The mechanism below Electron. Two X11 capturers running at once in one process is a likely cause, but I did not trace it.
• That the fix holds on a GitHub runner. That needs a nix-build.yml run.
• Whether the combined call also fails on a real X11 desktop with a window manager.
• The main-process callers that still combine both types (select-source, source restore) are unchanged. They use 0×0 thumbnails and are not on the CLI path.

Before CI can say anything, nix-build.yml on main must get past the build again. It currently fails on an npmDepsHash mismatch in nix/package.nix, which this PR does not touch.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved screen and window source detection on Linux X11 by enumerating each source type separately, helping prevent source enumeration from hanging.
  • Updated source enumeration checks to require successful results across all expected attempts.

[coderabbitai]

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📒 Files selected for processing (5)

• .github/workflows/nix-build.yml
• electron/ipc/desktopSourceTypes.test.ts
• electron/ipc/desktopSourceTypes.ts
• electron/ipc/handlers.ts
• src/cli/CliSourcesRunner.tsx

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📝 Walkthrough

Walkthrough

The IPC handler now enumerates source types separately on Linux unless the session indicates Wayland. The Nix smoke test now requires each of five enumeration attempts to return a display and requires export to succeed.

Changes

Desktop source enumeration

Layer / File(s)	Summary
Platform decision and per-type enumeration electron/ipc/desktopSourceTypes.ts, electron/ipc/desktopSourceTypes.test.ts	The helper selects separate enumeration on Linux outside Wayland. When enabled for multiple types, it calls getSources sequentially for each type and combines results in request order. Tests cover platform conditions, passthrough cases, ordering, and rejected calls.
IPC handler integration electron/ipc/handlers.ts, src/cli/CliSourcesRunner.tsx	The get-sources handler uses the helper and retains its existing deadline. CLI comments describe the X11/Electron 41 case and distinguish the renderer and handler timeouts.
Enumeration smoke-test gate .github/workflows/nix-build.yml	The smoke test treats an empty display result as a failed attempt. The final gate requires export and all five enumeration attempts to succeed.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant GetSourcesHandler
  participant getSourcesByType
  participant desktopCapturer
  GetSourcesHandler->>getSourcesByType: Pass options, getSources callback, and platform decision
  loop Each requested source type when separate enumeration is enabled
    getSourcesByType->>desktopCapturer: Call getSources with one source type
    desktopCapturer-->>getSourcesByType: Return sources for that type
  end
  getSourcesByType-->>GetSourcesHandler: Return combined sources

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Suggested reviewers: my-denia

Merge Risk: ⚪ Minimal · up to 39c02

This change makes screen enumeration on Linux X11 more reliable and tightens the CI smoke test. No concrete merge-blocking issue was found. The author notes validation on a real X11 desktop and a GitHub runner is still pending.

🚥 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 5 functions across 4 files. (1 skipped: 1…	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 and concisely describes the main change: separate screen and window enumeration on Linux X11.
Description check	✅ Passed	The description is detailed and on-topic. It includes the summary, related issue, reproduction data, testing results, limitations, and CI context. The template's Type of change, Release impact, Deskto…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

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 5 functions across 4 files. (1 skipped: 1 unsupported.)

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