Godot (as of version 4.4.1) doesn’t support the common alt+enter shortcut convention used in games. https://github.com/godotengine/godot-proposals/issues/1983

Someone posted a workaround, but I did not think their workaround was very good, in part because it uses the Input class instead of using the event parameter for the _input function. So, I came up with my own workaround:

extends Node
 
func _input(event) -> void:
	if event.is_action_pressed("toggle_fullscreen"):
		toggle_fullscreen()
 
static func toggle_fullscreen() -> void:
	var mode := DisplayServer.window_get_mode()
	match mode:
		DisplayServer.WINDOW_MODE_WINDOWED:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
		DisplayServer.WINDOW_MODE_MINIMIZED:
			pass
		DisplayServer.WINDOW_MODE_MAXIMIZED:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
		DisplayServer.WINDOW_MODE_FULLSCREEN:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)
		DisplayServer.WINDOW_MODE_EXCLUSIVE_FULLSCREEN:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)

Or, if you don’t want to configure an action:

extends Node
 
func _input(event: InputEvent) -> void:
	if event is InputEventKey:
		var key_event: InputEventKey = event
		handle_key_event(key_event)
 
static func handle_key_event(event: InputEventKey) -> void:
	if not event.pressed:
		return
 
	if event.alt_pressed and event.physical_keycode == KEY_ENTER:
		toggle_fullscreen()
 
static func toggle_fullscreen() -> void:
	var mode := DisplayServer.window_get_mode()
	match mode:
		DisplayServer.WINDOW_MODE_WINDOWED:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
		DisplayServer.WINDOW_MODE_MINIMIZED:
			pass
		DisplayServer.WINDOW_MODE_MAXIMIZED:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
		DisplayServer.WINDOW_MODE_FULLSCREEN:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)
		DisplayServer.WINDOW_MODE_EXCLUSIVE_FULLSCREEN:
			DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)

In either case, adding this script as a global autoload adds the shortcut convention to the game. I like to add it named as GlobalShortcut.