There are various places in our code that want to know if something is "hidden" in some sense (scrolled out of view, or clipped, etc.). Accessible::VisibilityState is one example. This code uses GetCrossDocParentFrame to walk the frame tree to the top of the top-level document. For oop-frames, that will no longer be possible.

The APZ machinery can probably help, but it may be necessary to maintain clipping information on oop-frames to continue to facilitate the checks that we do now. That's complicated by animation, transforms, etc. Perhaps at display list construction time in the parent we can send the latest information to embedded oop-frames?

I talked this over with rhunt and we concluded that a GFX person would be best positioned to work on this. Ryan will investigate and thinks this may be a good Fission M2 candidate.

I'm closing this in favor of bug 1519546.