Auto screen res was removed in one of the recent updates. The only way i know of sort of enabling borderless fullscreen is to start the game then press f11 twice, this will put the game into a pseudo-borderless mode but it will revert after changing settings or restarting the game.
Viewing post in Borderless Fullscreen?
I just tried doing this, but it just seemingly swapped between window and fullscreen. Alt tabbing still resulted in a black screen delay, unfortunately...
Of course, it's not the end of the world or game breaking, it's just a bit annoying that I can't seamlessly swap between windows. Hope this option gets added in a future update.
As I had mentioned, it's definitely possible, because that's how I play the game. It might be an option you need to toggle on, but it's not labelled as expected. I genuinely can't remember what I did to do it back then, but I've never had to change it, even through updates. I wish I had an answer.
I'll look into my settings and see.