I figure this would be really useful for a virtual pet style game. I keep reading the gamemaker forums that there's probably an extension that can do it. Is this the one?
Custom window frame for GameMaker games · By
If you only need to pin the window, that’s window_set_topmost
in the other, free extension. It can be used on its own or along with this/other extensions.
I also have an extension for custom window shapes.