System Buttons

[XpanD]

I've been using Immersive Full-Screen Mode by Pieter Pareit on my phone for a while, may be worth a shot. I can't post a link (some silly forum limitation), but just Google it. It's a paid app, but there may be free alternatives if you look for "immersive mode".

Be sure to toggle immersive mode ON (via the notification drawer thingy) before starting PPSSPP or any other program you want in proper full screen.