In the bad build of the window of comment#4, setting widget.windows.alternate_fullscreen_heuristics to false fix the issue. (the pref was added by Bug 1732517. And removed by Bug 1835851) So, I think this is regressed by Bug 1732517.
Bug 1842170 Comment 12 Edit History
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In the bad build of the window of comment#4, setting widget.windows.alternate_fullscreen_heuristics to false fix the issue. (the pref was added by Bug 1732517. And it is no longer available as it was removed due to Bug 1835851.) So, I think this is regressed by Bug 1732517.