fix(engine): apply compositor determinism flags in screenshot mode#865
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miguel-heygen wants to merge 1 commit into
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fix(engine): apply compositor determinism flags in screenshot mode#865miguel-heygen wants to merge 1 commit into
miguel-heygen wants to merge 1 commit into
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Chrome's compositor determinism flags (--run-all-compositor-stages-before-draw, --disable-threaded-animation, --enable-surface-synchronization, etc.) were previously only applied for BeginFrame mode on Linux. In screenshot mode — used on macOS/Windows — the compositor ran without these constraints, allowing Metal on Apple Silicon to accumulate state drift over sustained frame captures. This produced vertical layout shifts after ~12 seconds of rendering, reported on M1 machines. The shifts occurred because the compositor's threaded animation and surface synchronization pipelines could race with Page.captureScreenshot, and without --run-all-compositor-stages-before-draw, pending compositor stages weren't flushed before each screenshot. Split the flags into two groups: - BEGINFRAME_EXCLUSIVE_FLAGS: --enable-begin-frame-control and --deterministic-mode, which pause the compositor or freeze the clock and must NOT be used in screenshot mode - COMPOSITOR_DETERMINISM_FLAGS: the remaining 7 flags that enforce deterministic compositor behavior and are safe for all capture modes The compositor flags are now applied unconditionally in buildChromeArgs. Closes #828
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Summary
--run-all-compositor-stages-before-draw,--disable-threaded-animation,--enable-surface-synchronization, etc.) were only applied for BeginFrame mode (Linux). Screenshot mode (macOS/Windows) ran without them, allowing Metal's compositor to accumulate state drift over sustained frame captures--enable-begin-frame-control,--deterministic-mode) and 7 compositor-determinism flags that are safe for all modes. The 7 are now applied unconditionallyDetails
The compositor determinism flags enforce:
--run-all-compositor-stages-before-draw— flush all compositor stages before each draw/screenshot--disable-threaded-animation— animations process on main thread, not a separate compositor thread--enable-surface-synchronization— renderer and browser compositor surfaces stay in sync--disable-threaded-scrolling,--disable-checker-imaging,--disable-image-animation-resync,--disable-new-content-rendering-timeout— remove other sources of non-determinismWithout these, Chrome's Metal backend (Apple Silicon) could race between the compositor thread and
Page.captureScreenshot, accumulating vertical offset over hundreds of frames.Test plan
Closes #828