fix(capture-linux): measure the cursor against the window, not its mo…

[bosskopp]

Summary

Recording a window on Linux drew the cursor overlay in the wrong place for the whole take — a constant offset, present from the first frame, visible in a three-second clip. Full-screen capture was unaffected.

mutter never renegotiates the format for a window stream. It pins the stream to the window's monitor and carves the window out through SPA_META_VideoCrop, which can move on any buffer as the window does. The encoder already reads through that rect (Capture::read_origin), so the MP4 holds the window — but the portal reports the pointer in stream pixels, measured from the monitor's corner, and emit_sample normalised it against the negotiated format. A monitor-relative position divided by monitor dimensions, painted onto window-sized footage: wrong origin and wrong scale. For a 640×480 window at (100, 50) on a 1920×1080 monitor, a pointer at the window's centre serialised as 0.219, 0.269 instead of 0.5, 0.5.

Capture now remembers the rect it actually read — read_origin at the committed size, so it follows a window that moves and keeps the clamp that stops a shrunken window reading past the buffer — and content_rect() is the single place that answers "what does the file show". emit_sample subtracts that origin and reports that rect's dimensions, so the accumulator's x / width lands in the space frame_geometry.rs already assumes.

visible moves with it: it was tested against the monitor, so a pointer that had left the recorded window still reported visible: true.

Full-screen capture is byte-identical — no crop means the content rect is the stream. A cursor-only session opens no encoder and keeps normalising against the stream, which is correct there: its video comes from Electron, not from the helper.

Files touched

File	Change
electron/native/pipewire-capture/src/capture.rs	Capture::content tracked in stage(); content_rect() accessor
electron/native/pipewire-capture/src/main.rs	content_rect() helper; emit_sample measures against the content rect
electron/native-bridge/cursor/recording/pipeWireCursorAccumulator.ts	comment only — cx = x / width was already right
technical-documentation/architecture/recording.md	documents the crop/cursor coordinate contract

Related issue

Fixes #513

Type of change

• Bug fix
• Feature
• Enhancement
• Documentation
• Refactor / maintenance
• Performance
• Security

Release impact

• Patch
• Minor
• Major / breaking change
• No release note needed

Desktop impact

• Windows
• macOS
• Linux
• Installer / packaging
• Not platform-specific

Screenshots / video

Not included — the defect is a cursor-position offset in a screen recording, and a still frame cannot show where the pointer actually was. The numeric case in the summary (0.219, 0.269 vs 0.5, 0.5) is what the tests pin instead.

Testing

cargo test in electron/native/pipewire-capture — 69 pass (63 before, 6 new). Note that CI does not build or test this crate (the only cargo job in ci.yml is the macOS compositor), so this is the only automated signal on it.

New tests:

• capture::tests::the_content_rect_is_the_window_the_file_shows — initial rect, a window that moves mid-take, and the clamped out-of-buffer origin.
• cursor_sample_tests::a_window_capture_reports_coordinates_inside_the_window — the bug itself.
• cursor_sample_tests::a_full_screen_capture_reports_stream_coordinates — the no-regression case.
• cursor_sample_tests::a_pointer_outside_the_window_is_reported_invisible — both directions (past the far edge, and negative).
• cursor_sample_tests::nothing_is_emitted_before_the_format_is_known
• cursor_sample_tests::the_content_rect_prefers_the_encoder_once_it_has_started

Also run:

• cargo clippy --all-targets — no new warnings (the 6 existing ones are pre-existing dead-code notes).
• npx tsc --noEmit and npx tsc -p tsconfig.test.json --noEmit — clean.
• npm run lint — 14 warnings, identical to a clean main.
• npm run test — 2045 pass / 40 fail, identical counts to a clean main (pre-existing jsdom localStorage setup failures in src/components/**, unrelated to this change; verified by stashing and re-running).
• node scripts/check-docs.mjs — OK (31 files).
• npm run build:native:linux — the release helper builds and its startup probe reports {"cursorMetadataSupported":true,"pipewireVersion":"1.0.5","event":"ready"}.

Manual verification on Ubuntu / GNOME / Wayland (mutter, AMD): the v1.10.0 release AppImage was extracted and run against the rebuilt helper via OPENSCREEN_LINUX_CURSOR_HELPER_EXE, recording a window. The cursor lands where the pointer actually was. Full-screen recording re-checked for regressions.

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