Multiple answers. Windows Terminal users constantly had problems because WT is ... not great. Fullscreen mode fixes that. This also applies to some other lesser used terminals.
I wanted the out of the box experience to be smoother for more people, so made it the default. Scrollback mode will stay though for users who prefer it.
The codemode stuff/mcp has been explained to death at this point but I haven't seen anything around fullscreen.
(I don't have strong feelings either way, just curious)