commit ffdf088f9725dfe5d860be138d687ac8a3a8b4f5
author graouts@webkit.org <graouts@webkit.org@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Tue Nov 26 10:11:55 2019 +0000
committer graouts@webkit.org <graouts@webkit.org@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Tue Nov 26 10:11:55 2019 +0000
tree 12cc155aecbd1de591661382b76770b74d9ef1fa
parent 2f85d80d9229908a4acd79099c8b93e33e7c767e

[Web Animations] Layout of children of element with forwards-filling opacity animation may be incorrect after removal
https://bugs.webkit.org/show_bug.cgi?id=204602
<rdar://problem/45311541>

Reviewed by Antti Koivisto.

Source/WebCore:

Test: webanimations/child-layer-position-after-removal-of-animation-triggering-stacking-context-with-fill-forwards.html

In the case where we animate a property that affects whether an element establishes a stacking context, for instance `opacity`, the animation code,
specifically KeyframeEffect::apply(), forces a stacking context by setting setUsedZIndex(0) on the animated RenderStyle in case the element otherwise
has an "auto" z-index. This is required by the Web Animations specification (https://w3c.github.io/web-animations/#side-effects-section).

This means that a fill-forwards animation will still force the element to establish a stacking context after the animation is no longer "active". When
we remove such an animation, it may go from having a z-index to not having one, unless it had an explicit z-index provided through style. When this change
happens, RenderStyle::diff() will merely return "RepaintLayer" and thus will not foce a layout. However, updating the positions of child layers may be
necessary as the animation being removed may mean that there may not be a RenderLayer associated with that element's renderer anymore, and if that RenderLayer
had a position, then the position of child layers will no longer be correct.

Now, in the case where we destroy a layer in RenderLayerModelObject::styleDidChange(), we check whether the layer had a position before it is removed, and
update the position of child layers if it did.

* rendering/RenderLayerModelObject.cpp:
(WebCore::RenderLayerModelObject::styleDidChange):

LayoutTests:

Add a new ref test that checks that removing a forwards-filling animation that triggers a stacking context (for instance, animating opacity)
after it has completed from an element affecting layout yields the correct layout.

* webanimations/child-layer-position-after-removal-of-animation-triggering-stacking-context-with-fill-forwards-expected.html: Added.
* webanimations/child-layer-position-after-removal-of-animation-triggering-stacking-context-with-fill-forwards.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@252879 268f45cc-cd09-0410-ab3c-d52691b4dbfc