Helpfile under $window:
Note: You can also get the .w and .h properties by specifying: -1 for the width and height of the screen, -2 for the main mIRC window, and -3 for the MDI window where all other windows inside mIRC are displayed.
There aren't that
many useful undocumented features. Off the top of my head I can only think of 3 - $pi, $nickmode, and the use of ranges in $bvar(). Most of the things listed in threads about undocumented features are undocumented for a reason - they're either bugged or deprecated. And then there's plenty people say are undocumented where they've made the mistake like you just did with $window(-1) by simply not seeing it in the documentation.