Ok, this is a really odd one. Its part deux of bug 726545. There we had scroll bars pop up and cause invalidation when moving to the right. Here we set overflow hidden and move to the left and all is good, until the text completely disappears and only the background of the div remains (also white), at which point we start invalidating the remaining parts of the div. Awesome!
In this case, we get into a situation where nothing in the div is visible, so it gets no layer. Then its transform changes, InvalidateTransformLayer gets called, and finds no layer, so decides to invalidate the entire frame bounds. It's not immediately obvious to me how to fix this. But maybe that's just because I need sleep. Display-list-based-invalidation will fix it, of course.
I don't see any invalidation in this test with the fix for bug 725580 applied. (How long am I supposed to run it?) But I need to sleep too.
Waiting on mattwoodrow in the bug you asked me to depend on.
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