I suspect it's caused by the aspect ratio when WM tries to shrink down the window to the correct aspect ratio but it fails. I saw similar bugs for Gnome before but it was fixed. I think we may not set the aspect ratio for the GtkWindow at Gtk level as it already causes some issues like Bug 1584790 and linux version of Bug 1535437 but rather we may keep the aspect in mind when resizing the window by gecko.
Bug 1600414 Comment 7 Edit History
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I suspect it's caused by the aspect ratio when WM tries to shrink down the window to the correct aspect ratio but it fails. I saw similar bugs for Gnome before but it was fixed. I think we may not set the aspect ratio for the GtkWindow at Gtk level as it already causes some issues like Bug 1584790 and linux version of Bug 1535437 but rather we may keep the aspect on mind when resizing the window by gecko.