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This CL fixes an issue with scrollbar thumb drags being jittery. The
pointermove(s) are not Vsync aligned. When the first GSU is processed,
it gets queued in the compositor thread event queue. A second request
(to get a GSU for a pointermove) within the same frame will end up
calculating an incorrect delta (as ComputeThumbQuadRect would not have
accounted for the delta in the first GSU that was not yet dispatched).
This leads to generating a GSU with a delta that is larger than what it
is supposed to be. When these queued GSUs are finally dispatched, the
subsequent pointermove will make the thumb jump backward and this
problem continues on leading to a jittery thumb drag experience. To fix
this, we process only 1 GSU per frame when thumb drag is in progress.

Bug: 986174
Change-Id: Ied8103ffc447966fc15d0052c5295f896dcbcc29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1770088
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691973}