[Webkit-unassigned] [Bug 55257] Support creating compositing layers for scrollable frames and iframes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=55257





--- Comment #37 from Daniel Sievers <sievers at google.com>  2011-04-05 19:35:25 PST ---
Simon, 

I picked this up again today and looked at how this would work differently with respect to what you suggested when we chatted the other day.

With respect to your question to what causes the compositing to be re-evaluated when e.g. the iframe size changes: With my attached patch, the compositing layers will always be updated after layout (even if we previously were not compositing, because I set 'm_compositingDependendsOnGeometry').

About handling the overflow from the RenderIFrame: needsToBeComposited() is currently not called for RenderIFrames, because RenderIFrames usually don't have a RenderLayer. They only create a RenderLayer it if the child is compositing. That's why it plays nicely with the attached change.

Also it seems like the overflow would have to be detected by the child in one form or another anyway, because it has to happen after the child FrameView has completed layout.

About using 'RenderLayerCompositor::m_forceCompositing', that seems to work fine too. However, not sure what a good place to force it would be. Somewhere at the end of FrameView::layout() is called? It could probably go into FrameView::layout before updateCompositingLayers() is called after the actual layout, so that if the document changes and does not overflow anymore we can disable m_forceCompositing again (compositing will still have to be updated because we don't know if 'force' was the only reason to turn it on).

Otherwise, the graphics layers created by m_forceCompositing vs. using needsToBeComposited() for the RenderView's layer seem to be the same.
Speaking of which, I don't understand the '(inCompositingMode() && layer->isRootLayer())' part in needsToBeComposited(). Why does the root layer need a backing when it already creates a 'root platform layer' (the latter being what seems to be used in compositing)?

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