2038478 - Update the new snap target ids properly even if the destination is unchanged in SmoothScrollTo

Status: RESOLVED FIXED

(Core :: Panning and Zooming, defect, P2)

Description

[Hiroyuki Ikezoe (:hiro)]

The code in question is:

https://searchfox.org/firefox-main/rev/574d441bd2fdc78f8487aa317ea2f56aad28621f/gfx/layers/apz/src/AsyncPanZoomController.cpp#4270-4272

if (ConvertDestinationToDelta(aDestination.mPosition) == ParentLayerPoint()) {
  return;
}

If we have valid snap target ids, we need to use them for the next re-snapping opportunity.

This bug is one of the causes of test failures caused by D294084.

Comment 1

[Hiroyuki Ikezoe (:hiro)]

Attachment: Bug 2038478 - Update mLastSnapTargetIds when SmoothScrollTo exits early without scrolling. r?botond

When the destination passed to SmoothScrollTo is at the current scroll
position, the function returns early to avoid triggering a spurious
scrollend event. However, this also skips updating mLastSnapTargetIds,
so the snap target identifiers reported by the next CSSScrollSnapEvent
can be stale even though we just resolved to a snap target.

Move the destination's target IDs into mLastSnapTargetIds before the
early return so snap target tracking remains correct regardless of
whether the snap actually produces motion.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/a539841db290
https://hg.mozilla.org/integration/autoland/rev/22d48c07f481
Update mLastSnapTargetIds when SmoothScrollTo exits early without scrolling. r=botond

Comment 3

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/22d48c07f481