Created attachment 168600 [details] demo As shown in the figure, when using breeze's default window decorator, the maximized application does not fill the screen. After testing, this does not happen with other themes, and this does not happen with applications with self-painted window title bars. Condition.
Furthermore, it was caused by turning on "Draw border on maximized and titled windows". After turning off this option, the problem disappeared.
*** This bug has been marked as a duplicate of bug 485060 ***