Created attachment 203516 [details]
Content flowed in a region that has an animated transform becomes invisible if/when the transform causes it to render outside the region.
Repro: Open attached test case and observe the animation of the gray <div> (the one with BOO in it)
Expected: Animation plays and is visible at all times
Actual: The animation plays, but as soon as the gray div is entirely outside of the region (red dashed border), it becomes invisible. It becomes visible again once part of it are again "inside" the region
Created attachment 203528 [details]
Still repros on today's nightly (r151543)
Works fine in 168728, closing.