actually, also the minimize button does not works as another user already explained.
I’m not an expert developer but in my opinion any window that is resizable should have the maximize button (since it is a particular case of size). The minimize button should be available if the window is not modal.
But whatever the developer’s idea, if the button is there it should work!
Well this is odd. On my Ubuntu 23.10, I suddenly found only the “X”.
So I restarted the PC and the non functioning minimise and maximise buttons are back.
This smells of a windows manager bug.
The only Max & Min buttons I have are for the Project and Eight of the Editor screens (not the Plugin Manager).
The buttons displayed on @Andrea_Simeoni & @gmc examples do not exist on my version. Mint 21.3 using ubuntu 22.04.1 (my full version details are in an above post).
This is the KiCad Preferences dialog (which is displayed from the KiCad → Settings… menu, note the difference between the menu text and the dialog title) which has a working maximize button.
All of these are Modal dialogs as far as understand it.
I have been trying various dialogs and there seems to be a lot of variation as to whether or not there is a maximize button, and if there is one whether or not it works. I don’t think this is strictly a Ubuntu/Gnome issue.