Bug 90633 – Start off with overlap testing enabled when recursing into a stacking context layer

• Status: NEW
• Component: Layout and Rendering

[Simon Fraser (smfr)]

In RenderLayerCompositor::computeCompositingRequirements(), childState inherits the overlap testing state from the ancestor state (the logic being that if overlap testing is off, we need to keep it off when recursing into descendants).

However, I think we can turn overlap testing on for an overflow:hidden layer that is a stacking context.

[Simon Fraser (smfr)]

I don't think the current logic is correct. First, I think we can enable overlap testing when descending into a compositing subtree in more cases.

Secondly, I think here:

        // We're done processing an element that clips. The container can keep testing overlap.
        compositingState.m_testingOverlap = true;

we shouldn't be unconditionally re-enabling overlap testing; there may have been a 3d or animating layer in the context of that ancestor state.

Another way to approach this might be to "taint" the Regions in the overlap map when they contain a 3d or animating element; that tainting would propagate when regions are unioned.