Why mainwindow can still showmaximized() after setFixedSize()?

[nengluo]

The mainwindow has setFixedSize(250,316),but it can still showMaximized(). It seems like the mainwindow always has background. Please help me!

[nengluo]

The mainwindow's centralwidget has layout, in the layout i add another window. The mainwindow shows on the left-top corner of the screen while showmaximized(), and the other area of the screen is white color(background color). The mainwindow is inherited from BaseWindow which is finally inherited from QMainWindow.

[ChrisW67]

Qt does that because you asked it to do contradictory things and it has to choose one.

If you do not want the window maximized then do not call showMaximized() and hide the maximize widget (https://doc.qt.io/qt-5/qwidget.html#windowFlags-prop) to make it difficult for the user to do it.