1597595 - [wpt-sync] Sync PR 20312 - Store ScrollAnchor adjustments outside of ScrollableArea

Status: RESOLVED INVALID

(Testing :: web-platform-tests, task, P4)

Description

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Sync web-platform-tests PR 20312 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/20312
Details from upstream follow.

Chris Harrelson <[e-mail]> wrote:

Store ScrollAnchor adjustments outside of ScrollableArea

This avoids issues where no-op sequences of layout updates and forced
layouts may cause user scrolls to be lost due to clamping. For example,
a forced layout may cause an anchor element to move down and also increase
scroll height; then that layout may be undone and cause the opposite.
If scroll anchor adjustments are not stored outside of ScrollableArea,
then the "undone" part will lose information about the true user scroll.

With this technique, a "no-op" rendering update that involves two mutations
to layout that are the opposite of each other, and that have impact
on scrollable area sizing, will now have a no-op impact on final scroll
offset.

Bug: 997502,988991,1022615
Change-Id: Ibd567005443dc7b0449ad97a0d886e8446cf876c

Reviewed-on: https://chromium-review.googlesource.com/1907598
WPT-Export-Revision: 4e5f9aaef4c18815da99c273403c17b8be371c60