2D canvas contexts started publishing partial frames after canvas
rasterization moved into the Compositor. WebContent still splits large
recorded command lists after 64 commands, but every split batch was sent
through the same compositor path as the end-of-frame flush. The
compositor replayed each batch into the DrawCanvas source surface,
so a pending present could sample a canvas after clear and before the
rest of the next frame had been drawn. Canvas-heavy pages such as
slither.com then flickered between partial and complete frames.
Carry an explicit commit bit with 2D canvas command updates. Non-commit
batches now update a hidden working canvas in the Compositor, while the
display-list-visible surface keeps the last committed canvas contents.
The end-of-frame canvas preparation sends the commit boundary, including
the empty-commit case needed when the auto-flush consumed all recorded
commands before prepare_for_compositing() runs.
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