I've verified this problem on Windows and Linux, but it appears that the doorhanger already appears in the correct place on macOS. The underlying problem seems to be a 4 pixel margin that the doorhanger is given that pushes it below the bottom of the chrome. I believe that the 4 pixels are needed to display a shadow "under" the doorhanger. But it looks like that wasn't done on macOS, for some reason. I should have a patch up for this shortly.
Bug 1718011 Comment 7 Edit History
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I've verified this problem on Windows and Linux, but it looks like the doorhanger already appears in the correct place on macOS. The underlying problem seems to be a 4 pixel margin that the doorhanger is given that pushes it below the bottom of the chrome. I believe that the 4 pixels are needed to display a shadow "under" the doorhanger. But it looks like that wasn't done on macOS, for some reason. I should have a patch up for this shortly.