Sotaro also helped me tracked down another secondary bug (and most likely the primary cause of this), related to a clear region optimization where we collapse a rectangular clear into a single row. This can clear too much of the buffer if the framebuffer in question is actually only a sub-region of a much larger buffer, and in that case we might clear areas outside the sub-region erroneously.
Bug 1698067 Comment 18 Edit History
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Sotaro also helped me tracked down another secondary bug (and most likely the primary cause of this, whereas the flushing issue is probably theoretical and not the real cause), related to a clear region optimization where we collapse a rectangular clear into a single row. This can clear too much of the buffer if the framebuffer in question is actually only a sub-region of a much larger buffer, and in that case we might clear areas outside the sub-region erroneously.