Here's the code I used for the fullscreen toggle. I hope it helps!
func _on_fullscreen_toggled(toggled_on: bool) -> void: if toggled_on: DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN) else: DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)
Sorry about the audio, I'm aware of the issues thanks to other comments, but I'm planning on fixing it after the jam. Thank you for playing! <3