Thank you for your answer but the issue i have is not the drawing of lines but the size of the parent.
I want to build a convoy of ships and want to test if the whole convoy has left the screen.
I also want to be able to programatically fill the convoy with ships and do not want to edit every convoys "size box" by hand.
If i could figure out a way to get the size (a Rect2D that encloses all child elements) i could solve my issue with code in process() or with a VisabilityNotifier() but for both i need the size.
In the past i've solved this kind of thing with get_rect() but this seems to be gone now.