commit 712c3cf3ed8201420acf23f760eaa34be20781cd
author Majid Valipour <majidvp@chromium.org> Tue May 07 21:51:38 2019
committer Commit Bot <commit-bot@chromium.org> Tue May 07 21:51:38 2019
tree a9b9547b4eead43c32f222221b35ac0b85bcd8df
parent 7181ee07e464941e2aa3889dd19c7c3ea5eebd8d

Correctly handle scroll-snap-type changes to 'none'


Previously when a scroll container's snap type is changed to 'none' its
data was discarded including all of its snap areas. However this is
incorrect. Because while the snap type is 'none', the element is still
a scroll container which per spec [1] means  that is should continue to
captures the snap areas in its subtree for whom it is the nearest
ancestor scroll container . The only difference is that it no longer
snaps.

The fix is that we no longer remove the snap container data just
because is has a 'none' snap type and instead keep it and its snap
areas. But we check the snap type before performing any snap.

To ensure this does not introduce any performance regression, this CL
also includes an optimization where we avoid re-calculating
snap_container_data when the snap type is 'none'. So keeping these snap
data should not be cheap.

Note that there is another problem where if the current snap container
is no longer a scroll container (e.g., overflow: scroll => overflow:
visible) we release its snap areas and they become "orphan". But if we
are to do this correctly, we should re-assign these areas to the next
stroller in the chain. Similarly when an element becomes a scroll
container, it can potentially take over snap areas from its parent snap
container.


This patch does not address that situation yet but fixes the easier
problem.

[1] https://drafts.csswg.org/css-scroll-snap/#overview

Bug: 953575
Test:
 - wpt/css/css-scroll-snap/scroll-snap-type-change.html => Changing snap-type should work correctly
 - wpt/css/css-scroll-snap/scroll-snap-type.html => Add a specific test for type 'none' to ensure it does not snap

Change-Id: Ie493ad68ecba818ed41c0ee103ccf44725ff6e3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589899
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657460}