How do I keep window maximized

In the upper right corner of a Window, there are three buttons.

1 The leftmost button is an "_" (underline). You click that to totally minimize a window.

2 The rightmost button is an "X". You click that to close a window.

3 The middle button is either a single square with a thick bar over it. That's supposed to represent a maximized window. It toggles between a "two windows" icon, one window in front of the other -- meaning a window that's not maximized. If you want the window to be always maximized, make sure you see the "two windows" icon.