Hi all,
I know there are a couple different ways to get full screen but I found one that I think works the best.
This video is for Diablo 2 full screen but it works perfect for P99 and you don't have to resize your desktop:
https://youtu.be/6R-mC7i8fVs
When you alt/tab out of game, just hit alt/enter (once or twice) when you get back in to get full screen back.
Have fun!