We would need to explicitly trigger frame reconstruction when we move an element into or out from the top layer once we remove the usage of :-moz-full-screen-ancestor. Since elements in top layer are direct children of the viewport (considering only fullscreen case), we generally want to reconstruct the frame to change its geometry parent. This can normally be triggered by the change of "position" property. Also, elements originally with "position: fixed" don't need a reconstruction, all good. However, the issue is that, if an element has "position: fixed", but caught by a transformed ancestor, that path may not work. Currently, this issue doesn't matter because we have a UA stylesheet rule states: > :-moz-full-screen-ancestor { transform: initial !important; } which triggers the frame reconstruction for us when any transformed ancestor catch the fullscreen element. But when we remove this rule, we will need to face this issue again.

We may do that change in RestyleManager::ContentStateChanged().

For the same reason, we need to explicitly set the geometric parent of the frame to viewport in nsCSSFrameConstructor (should be done in ::AddChild() and ::GetGeometricParent()) when we handle this bug.