So in the Plexamp client for Mac the titlebar is hidden, but the Close and Minimize buttons can still be clicked. This can cause issues since the application doesn’t quit when its main window is closed:
What version of Plexamp do you have installed and on what version of macOS?
I’m running Plexamp 3.1.1 on macOS 10.15.6 and can not reproduce this – and believe me… I tried hard … when getting to the left, at some point Plex will consider this a click on the “recent plays” header and open that view.