mContentRect on FrameMetrics seems pretty useless. Not only can it be calculated from mCSSContentRect, but it's also not always completely up to date. We should remove it.

For the record, I also think that mContentRect is being calculated incorrectly in APZC. At [1], mContentRect is assigned a value that is the CSS page size /divided by/ the resolution. All the code that uses this, and the other bits at [2] that set this value all do CSS page size /multiplied by/ the resolution (or equivalent). I'm building a patch to get rid of this less-than-useless variable as it caused me to waste most of my day. [1] https://hg.mozilla.org/mozilla-central/file/58bf49390ed4/gfx/layers/ipc/AsyncPanZoomController.cpp#l835 [2] https://hg.mozilla.org/mozilla-central/file/58bf49390ed4/layout/base/nsDisplayList.cpp#l664

Fairly confident I updated all the calculations appropriately. Try push in progress at https://tbpl.mozilla.org/?tree=Try&rev=37616b14a0f5