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fix: capture screens with ScreenCaptureKit on macOS - #354
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A VideoDesktopSource capturing a screen on macOS always used ScreenCapturerMac, since WebRTC tries ScreenCaptureKit only when allow_sck_capturer is set. That capturer takes its first frame from a display snapshot and afterwards only copies what its CGDisplayStream reports as changed, so when the stream stops calling back, it keeps sending the first frame, with a live cursor composited on top (#344). Screens are now captured with ScreenCaptureKit on macOS 14 and newer, and with ScreenCapturerMac as before on older releases. ScreenCaptureKit is told the source's frame rate, as it otherwise captures at the display refresh rate. Source lists stay with ScreenCapturerMac: without its system picker ScreenCaptureKit lists no screens, and both name a screen by its display ID. Window capture is unchanged. A new test switches a window of its own between black and white while the screen is captured, and checks the frames follow.
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Addresses #344: screen capture on macOS freezing on its first frame while the cursor keeps moving.
Screens are captured with ScreenCaptureKit on macOS 14 and newer
VideoDesktopSourcealways gotScreenCapturerMacon macOS, because WebRTC tries ScreenCaptureKit only whenallow_sck_captureris set, and it never was.ScreenCapturerMactakes its first frame from a display snapshot and afterwards only copies in what its CGDisplayStream reports as changed. When that stream stops calling back, it keeps delivering the first frame asSUCCESS, andDesktopAndCursorComposerdraws a live cursor over it: the symptom in the issue.VideoTrackDesktopSource::capture()now setsallow_sck_captureron macOS. On releases before 14,CreateScreenCapturerSckreturns null and WebRTC falls back toScreenCapturerMacas before. The capturer is also given the source's frame rate, since ScreenCaptureKit otherwise captures at the display refresh rate.What stays as it was:
ScreenCapturer.getDesktopSources()): without its system picker, ScreenCaptureKit lists no screens, soDesktopCapturerkeepsScreenCapturerMac. Both name a screen by itsCGDirectDisplayID, so the listed IDs select the same screen.WindowCapturerMacis still used.One behavioural difference, from reading WebRTC rather than testing it: without screen recording permission, ScreenCaptureKit fails with
ERROR_PERMANENTand the capture stops, whereScreenCapturerMacdelivered the wallpaper and the app's own windows.Reproduction
Not reproduced. On macOS 14.5 (Apple silicon, screen recording permission granted), the unfixed build followed a window switching between black and white in another process every 400 ms, in three 12 s runs, and window capture followed it too. The reporter's macOS version isn't known yet.
Testing
macOS 14.5 arm64:
mvn -pl webrtc testpasses (284 tests, 0 failures, 15 skipped), with the desktop capture tests run, not skipped.With the fix, the WebRTC log shows
ScreenCapturerSckcapturing the display listed byScreenCapturerMac. Frames follow the same switching window as before, the first one arriving 120 to 230 ms afterstart(), at the right size and colours, with a single cursor.VideoDesktopSourceTests.screenFramesFollowScreen(macOS only) switches a window of its own between black and white while the screen is captured, and fails if the frames don't follow. It passes on this Mac with either capturer, so it guards against a frozen capture rather than reproducing #344.