I can reproduce this locally. I wonder if it might be related to the Clear rect primitive that gets used on mac, being interfered with. If we only see the bug on mac, that seems likely to be related.
Bug 1585898 Comment 4 Edit History
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I can reproduce this locally. I wonder if it might be related to the Clear rect primitive that gets used on mac, being interfered with by picture cache slices. If we only see the bug on mac, that seems likely to be related.