Source engine pwned.

So after the 90th time of going insane from source engine games (tf2 this time) going batshit loco when you alt-tab or a chat window pops up or you have 2 monitors or ANYTHING I actually went looking for an answer, and found it.
Make a shortcut for the game.
Edit the shortcut and add the following command line switches (except your monitor res ofc):

 -window 1920 1200 -noborder

This does as you’d expect and runs the game in a borderless window. It might incur a small performance hit, but alt-tab is now instant.

Praise the gaming gods.