At large sized ultrawide with a normal viewing distance you want the panel to have a slight curve as the horisontal lines will appear optically straight to your eyes. I'm looking at my 34" curved Mii 1440p secondary display right now and the UI appears perfectly straight to me.Wish this wasn't curved ...
(And I was a staunch anti-curved display before I got mine on a whim to play 144hz games on the old wintendo)