Game keeps switching to windowed mode

My game will start in windowed mode. I keep setting my game to go fullscreen. 15 seconds later it will change back to windowed mode and keep going in and out of full screen and windowed mode. It will also change to windowed mode any time I portal to a new area. How do I make this stop? I've been playing a long time and this has never happened before.

My game also starts windowed always, though i just switch it to fullscreen and play (so it's just annoying, no more). Still, wanted to have it fixed.

(copypasta from my reply in another thread about the same/similar issue)

I was having the same issue, and i believe i have found a fix (or a workaround i guess would be more accurate).

Steps:
0. Start PoE. Despite your config file indicating fullscreen, it starts in windowed mode, with a custom resolution.

1. Go into options and select windowed mode. Change the resolution to a coarser one, such that it no longer is a custom resolution (for my 1920x1080 monitor i had to select 1600x900 to get it to work; <number>x1024 or any larger y-value would change it to a custom resolution <number>x1001).

2. Change back to fullscreen.

3. Exit PoE.

4. Start PoE. It now starts in Fullscreen, as desired.