close/minimise/full screen buttons missing from app

I have this issue affecting only one app, Plexamp, on my M1 Mac mini, with Big Sur 11.2.3. The red, green and yellow buttons have just.. gone, and as the app is just a port from the iOS app to macOS, there are none of the regular preferences or view options available. I cannot resize the window by manually dragging from the corners, all I can do is go from full screen to minimised to the Dock. I have tried deleting the .plist file for the app but this did not achieve anything.