GitLab

Bug:
When a thumb drag is in progress, the scroller will jump every time the
scroller length changes. This happens because the pointer and the thumb
always try to stay in sync with each other. Main thread scrollbars dont
have this issue as they allow the thumb and pointer to get out of sync.

Fix:
If the scroller length increases mid thumb drag, we record the
"displacement" delta and early out. This delta is then applied to every
subsequent pointermove to ensure that that scroller retains its offset
even when the scroller length changes. The downside of this approach is
that the thumb and the pointer will get out of sync but the upside is
that the scroller won't suddenly jump.

Bug: 1067518
Change-Id: I773335db538b8595e96464f3306d2df9e82a58f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134923
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759288}