[Webkit-unassigned] [Bug 164949] -webkit-overflow-scrolling: touch completely breaks 3D perspective functionality

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=164949

Ali Juma <ajuma at chromium.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ajuma at chromium.org

--- Comment #5 from Ali Juma <ajuma at chromium.org> ---
(In reply to Frédéric Wang (:fredw) from comment #4)
> Some preliminary debugging: When webkit-overflow-scrolling is absent
> RenderLayerCompositor::computeCompositingRequirements will execute
> 
>     if (!willBeComposited && canBeComposited(layer)
>         && requiresCompositingForIndirectReason(layer.renderer(),
> childState.subtreeIsCompositing, anyDescendantHas3DTransform,
> indirectCompositingReason)) {
>         layer.setIndirectCompositingReason(indirectCompositingReason);
>         childState.compositingAncestor = &layer;
>         overlapMap.pushCompositingContainer();
>         addToOverlapMapRecursive(overlapMap, layer);
>         willBeComposited = true;
>     }
> 
> where requiresCompositingForIndirectReason returns true because of the
> perspective property on the overflow node and the 3D transform on its
> descendants. If the overflow node has overflow-scrolling:touch,
> willBeComposited is however set to true at the top of the function so this
> conditional is never executed. I'm not yet sure how/whether this is causing
> the bug, though.

I'd suggest looking at the corresponding RenderLayerBacking to see which GraphicsLayers are created, and which of these GraphicsLayers have preserves3D() true. Probably in the overflow-scrolling:touch case, there's a GraphicsLayer which ends up flattening the transform (because of preserves3D() being false), so the Z component ends up having no visual effect.

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