This shows up in test_transformed_scrolling_repaints_2 when the patches from bug 788044 are applied. We snap the scroll to the nearest (scaled) pixel bounds, and then convert that to app units. This doesn't correspond directly, so we round to the nearest whole app unit. We computing the visible area of the layer we can end up with values like ~X.999 in scaled layer pixels because of this rounding. This value rounds down to X, not X+1 when ScaleToOutside pixels is used. This only happens when we're directly on a layer pixel boundary, so varies based on scroll position.

The patches in bug 777194 fix test_transformed_scrolling_repaints_2 on all platforms, and test_transformed_scrolling_repaints on 64-bit Linux. So, this bug only affects test_transformed_scrolling_repaints on 32-bit Linux now.