Version: 55.0.2860.0 (Developer Build) (64-bit)
OS: All

What steps will reproduce the problem?
(1) Load the attached file with --enable-blink-features=GeometryInterfaces,CompositorWorker
(2) Scroll the page

What is the expected output?
Both of the boxes should stay stuck to the top of the viewport.

What do you see instead?
The box on the right seems to lag behind the scroll position. What is happening is every time the CompositorWorker script sets a transform it is sent back to the main thread, which then commits the new transform to the CompositorWorker in the next frame. This commit happens after the CompositorWorker has applied the transform for the next frame and clobbers that value. We then run the animation frame function again but the script sees the same scroll offset and so it doesn't update the transform of the box on the right.

We should prevent the updates from previous CompositorWorker frames from clobbering later ones. We can probably do this by protecting any value that has been updated in the current frame by a CompositorWorker callback.

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