Will this run 3440x1440? On steam the entire screen wasn't displayed.

I'm sure this will run.

I had the same problem with the same resolution. I found this from HustlinTom- "No idea if Bethesda did fix it or not, (leaning towards not) but if you just want to get Skyrim in Ultrawide aspect ratio mod-free, go into your Documents folder, then My Games, Skyrim (whichever edition), and open up SkyrimPrefs.ini in notepad/your preferred text editor. Replace 'iSize H' with 1440, and 'iSize W' with 3440 and save. The Launcher respects whatever is entered in the .ini file unless you change the resolution from inside the launcher itself. " I ended up going with a res of 'isize H' 2800 and 'isize W' 1410. It does not fill the whole screen, a little more them 3/4. but you can see all the screen even the bottom.

Have you tried alt+enter?

Which game?

Skyrim V

https://www.nexusmods.com/skyrimspecialedition/mods/1778

This mod should enable that to work.