Sometimes the top bar icons (battery icon, wifi, sound etc...) are going out of their boundary at the top bar. when doing so they take over any maximized window top bar (e.g. browser, file explorer or any other app). By doing so it prevents the user from reaching the X, resize, and minimize buttons at the top right bar of the app.
Sometimes the app menu on that same top bar is exceeding as well causing the same problem on the left side of the screen.
It happened several times, but i cant seem to notice what cause it...
The problem also disappear quite easily, but the only time i noticed something that stopped it is when i toggled off the "application menu" option in "Extensions" - i.e. made any structure change in the top bar.
I'm using ubuntu 20.04 LTS "focal fossa".
I use a workspace indicator, and a horizontal workspace structure which i both toggled on from "Extensions".
This sounds like bug 1877288. Please also try disabling the 'ubuntu-dock' extension.