Hello!
The extension doesn’t try to perfectly mimic the Windows title bars/borders because the most reliable way to do that is to have a secondary window wrap around the game window, which is what Window Freeze Fix did, and which is why it had so many oddities - although nested windows are a core feature of Windows, plenty tools don’t expect games and software to use this or don’t handle the various edge cases.
Fullscreen transitions are done using gameframe_set_fullscreen.