As a result of bug 1465616, fixed position elements aren't painted completely. Steps to reproduce: 1. Using Firefox for Android 63.0a1, open https://searchfox.org. 2. Notice that only the first half (or so) of the search bar is visible and the rest is missing. This is caused by how fixed position elements are sized: until bug 1465616 landed, they were sized using the SPC-SPS [1] (when it was set, which seemed to be always), but now they're sized using the visible rect [2] (unless the SPC-SPS isn't set, which, again, seems to be never). Fixed position elements can now be async scrolled, so using the SPC-SPS is no longer viable. The solution is to use the size of the layout viewport (which is given by |aFrame->GetParent()->GetSize()|). [1] https://searchfox.org/mozilla-central/rev/bf18fe089c7c424c4440a621214bdc3d81867ca9/layout/painting/nsDisplayList.h#1465-1482 [2] https://searchfox.org/mozilla-central/rev/8384a6519437f5eefbe522196f9ddf5c8b1d3fb4/layout/painting/nsDisplayList.h#1465-1475

Abandoning this since bug 1465616 was backed out.