This is probably my fault because recently I rewrote all the window creation code to support runtime fullscreen switching. I’ll take a look.
EDIT: but wait, have you tried passing the properties as the parameter to open_window()? (Although it should be technically possible to do it on the fly so I’ll take a look anyway)
First, you are actually passing the return value of set_undecorated() which is void. Second, you want to use the overload that accepts properties, so fill in a flags value.
EDIT: We should still make it possible to undecorate a window on the fly, since Windows can do that. Since the mechanism is there for the fullscreen switching it would be trivial to add.