So idk if this has been mentioned, but I dragged the window up to the top cuz I thought it would make it fullscreen but the window just moved out of the screen.
Sadly the custom title bar is not a 100% fully featured thing when the window is borderless in Godot, hence much of the anticipated basic functionality of windows needs to be reimplemented. Thank you for reporting the issue and luckily this is one that will be fixed in the next update. It won’t make the window fullscreen when you drag it to the top, but there are now limits in place. Created by the current size of the window.