I can't reproduce this on Linux with HW-WR or SW-WR. Interestingly I note that Chrome doesn't create the scrollbar that we see on Firefox with the attached test case.
Bug 1768779 Comment 4 Edit History
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I can't reproduce the screen glitches on Linux with HW-WR or SW-WR. The flashing I see although I'm not entirely sure what is supposed to render. I tried using Chrome as a reference, but it doesn't create the scrollbar that we see on Firefox with the attached test case.