It seems that 77.0a1 (2020-04-28) was affected by the outside boundaries, but the latest 77 (2020-05-03) seems to be having the same behavior as 76 which they are affected by the scrollbar overlap.
Bug 1633708 Comment 5 Edit History
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It seems that 77.0a1 (2020-04-28) was affected by the outside boundaries, but the latest 77 (2020-05-03) seems to be having the same behavior as 76, on which they are affected by the scrollbar overlap.