Positioned, compositing child jumps around as opacity transition runs

[Simon Fraser (smfr)]

Created attachment 38643 [details]
Testcase

The attached testcase shows a bug where the inner child renders in the wrong place while the opacity transition is running, when accelerated compositing is enabled.

[Simon Fraser (smfr)]

<rdar://problem/7173461>

[Simon Fraser (smfr)]

Created attachment 38765 [details]
Simpler testcase

[Simon Fraser (smfr)]

Created attachment 38767 [details]
Patch, testcases, changelogs

Comment on attachment 38767 [details]
Patch, testcases, changelogs

> +    for ( ; curr && !isPositionedContainer(curr); curr = curr->parent())
> +    { }

Is this the WebKit style convention for empty loop bodies? It seems strange.

>      for ( ; curr && !curr->renderer()->isRenderView() && !curr->transform(); curr = curr->parent())
> -        { }
> +    { }

Ditto.

> +            RenderLayer* positionedAncestor = enclosingPositionedAncestor();

I think you can call parentLayer->enclosingPositionedAncestor() instead.

> +            int thisX = 0, thisY = 0;

Please define each variable on a separate line.

> +            convertToLayerCoords(positionedAncestor, thisX, thisY);
> +            
> +            int ancestorX = 0, ancestorY = 0;

Separate lines, please.

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/47900