Created attachment 214199 [details]
The testcase shows a bug (missing background image on the middle div) which I think happens because when we determine that an element can use the solid color optimization, we omit to check to see if any descendants render into it.
A couple of things here:
1. I think this boolean logic in RenderLayerCompositor::requiresOwnBackingStore() is wrong:
but fixing that would prevent having nested "paintsIntoCompositedAncestor()" layers.
2. isSimpleContainerCompositingLayer() gives the wrong answer if descendant compositing layers are "paints into ancestor" layers.
Fixed via bug 133065.
*** This bug has been marked as a duplicate of bug 133065 ***