A recent State of the Game for Torchlight III touched on UI scaling, widescreen, and more.
The team outlined that they’ve decided to add a UI scaling option to the menus. It works pretty much as it does in other games. Changing the slider will simply affect the scale of the UI…at least for some time. apparently the UI “wigged out” but fortunately, it was fixed.
Additionally, widescreen and Safe Adjust settings are coming soon. Specifically regarding the widescreen option,
“Additionally, after another round of Options debate for widescreen players, we are working to add a "pillar box" option with black bars on the side so you can play the game on an ultra-wide with full in-game height. This will let you choose between full (ultra) width but squished in-game height or full in-game height with black bars on the sides. This is also not quite ready yet, but we hope providing more options as opposed to less will allow players to enjoy the frontier as they prefer.”
You can check out the full state of the game right here.