I just had the same issue, where I could not leave fullscreen properly or move the window at all.
The fix for me was to change a part in "C:\Users\'UserName'\AppData\Local\PlexMediaServer\storage.json"
To be more specific I changed this
"state":{"geometry":{"height":1043,"width":1666,"x":7,"y":0}
to
"state":{"geometry":{"height":1043,"width":1666,"x":0,"y":0}
You can probably even set it to this, which will set the window to size to default on next application start
"state":{"geometry":{"height":0,"width":0,"x":0,"y":0}
