I had the same problem with my ultrawide and I eventually (just now) figured how to force the game into windowed mode on startup automatically. Didn't find a better answer anywhere here. Follow these steps to utilize your whole ultrawide screen.
1: Go into your game folder, right click the .exe file (the game application), and create a shortcut (under: show more options)
2: Right Click the new shortcut icon and go to properties
3: In the text box to the right of ' Target: ' leave all the text and add the following to the right of the last quotation mark:
-screen-fullscreen 0 -screen-height 1400 -screen-width 2560
It should look something like this: "C:\blah\blah...lablah" -screen-fullscreen 0 -screen-height 1400 -screen-width 2560
4: Click Apply and then click OK
5: Start the game with the newly created shortcut.
The game should now start in Windowed mode and is playable that way. All menus should be fine. Changing the windowed resolution (Title page, Options) now changes the window size. Changing the resolution under options messes with menus and stuff (menus too big and trails off screen, unusable, can't change resolution, close the game and re-open the shortcut). Your in-game resolution should be set to 3440*1440(21:9). Going into fullscreen mode fills the sides with black bars. Alternatively, you can manually adjust your computer to run in a lower resolution to match the game's resolution instead of all this every time you play.
Don't do that. You bought an ultrawide screen. Use it.Before proceeding, adjust your game options now, (ie: language, quality, sound). If you want to use 'Edit' 'Options' and 'Workshop' from the title page, do it now, or revert width back to 2560 and re-open the game.
To Use the entire ultrawide screen while playing
6: Set the in game resolution, under options at the title page, to the highest it can go 3440*1440(21:9) and exit the game.
7: Re-open the properties of the shortcut and match the height and width to the max of your ultrawide screen: 1440*5120.
-screen-fullscreen 0 -screen-height 1440 -screen-width 5120
8: Click Apply and then click OK.
9: Start the game with the newly edited shortcut.
10: You can now switch over to full screen from window mode to full screen (alt+enter) with minor *issues, but it is very much playable.
*The Load and Save menus are slightly enlarged, clipping off the top option, autosave, and most of save 1. You can still click the save file 1 to load. (I never use autosave anyway)
*Warp to stand-by spot and Warp to player's spot buttons, under status, are mostly hidden by the backpack, equipment, and status icons, but are still clickable, barely.
*Map is big. Learn to scroll and right click to move map. "M" opens and closes map.
*In game options, under settings, has the top option, 'language,' cut off.
Things that get funky after step 7
*Title menu is enlarged, making the 'Edit' 'Options' and 'workshop' sub-menus not functional.
Window mode: 1440*2560
Fullscreen 1440*5120
