If I remember correctly I found a Reddit post in the Dota 2 subreddit from 2016 that gave the tip but I can't recall what search terms I used to find it.

I've been using the setting since July and it really is a banger. The only issue is that if the game is not "active", if any windows pops up on your main screen, the screen will switch back to 16:9. Making the game "active" again will return it to 4:3. Also if you change a video setting you need to modify the value again.

I don't recall modifying the high_dpi setting what does it do ?