The only way to set a window in fullscreen mode is to press f11 in mIRC. But if the alias f11 is defined by a script, then f11 won't make a window fullscreen anymore, and this is a problem.
I'd like to see a new switch for /window to make a window fullscreen.

Also mentioning my related suggestion
https://forums.mirc.com/ubbthreads.php/topics/269948/f11-tooltip

Some kind of tooltip or message to status window would be helpful because the full mode can freak people out if they get there without realizing what it is. The menu/logo/etc are all gone. OMG my windows taskbar is gone too!