This isn't a recent regression; I think it dates back to when we made these popups be "in-content" rather than being separate OS-level dialogs. I can repro in 2021-04-01, using STR from comment 3. I cannot repro in 2021-02-01 because the confirm-quit dialog seems to appear as an actual OS-level popup-window (locked to a position in the center of the initiating window). I can't easily further-bisect in that range since mozregression-launched Firefox refuses to show me the Ctrl-Q popup even if I ask it to, but it looks like this probably came from the `proton-modals` work, possibly bug 1685313 or something related to that.
Bug 1880685 Comment 4 Edit History
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This isn't a recent regression; I think it dates back to when we made these popups be styled content rather than being separate OS-level dialogs. I can repro in 2021-04-01, using STR from comment 3. I cannot repro in 2021-02-01 because the confirm-quit dialog seems to appear as an actual OS-level popup-window (locked to a position in the center of the initiating window). I can't easily further-bisect in that range since mozregression-launched Firefox refuses to show me the Ctrl-Q popup even if I ask it to, but it looks like this probably came from the `proton-modals` work, possibly bug 1685313 or something related to that.