This guide will teach you how to change your resolution to 3440x1440 (or really anything else) without the use of mods.

Before changing the resolution, we need to do just a couple of things.

1. Run the game, and open the options in the launcher
   
2. Uncheck 'Borderless' and 'Windowed'
   
3. Change the graphical settings here to fit your needs. (Don't turn on the weapon debris)
   
4. Save the settings and open the game
   
5. Once at the main menu, just quit to desktop

Note: If you want to change the UI colour, do it before closing the game. You won't be able to change it as easily later on.

Now we can change the resolution.

1. Open a terminal and CD to the Steam directory.
   For me it would look like:
   cd /mnt/games_drive/SteamLibrary
2. From here CD into the following folders.
   
   cd steamapps/compatdata/377160/pfx/drive_c/users/steamuser/Documents/My Games/Fallout4
3. Now open 'FalloutPrefs.ini' with a text editor.
   
4. Search for 'ISize' and you should see 'ISize H=600' and 'ISize W=800'
   Note: the numbers may be different than mine, it doesn't matter as we are going to change them.
   
5. Change 'ISize H' to your Height and 'ISize W' to your width.
   For 3440x1440 resolution, it will look like:
   
   iSize H=1440
   iSize W=3440
6. Find and check the settings below and make sure they match mine.
   
   bMaximizeWindow=0 bBorderless=0 bFull Screen=1
   Note: You might also want to set 'iPresentInterval' to 0 if you're having lag issues, since that fixed my lag.
   
7. If everything is good, save and exit back to terminal.
   
8. Set the file as read-only, so it doesn't randomly change.
   
   chmod -w Fallout4Prefs.ini
9. Open the game and you should be in 3440x1440

Since the game wasn't meant to support 21:9 resolutions, you will see some issues with the UI being incredibly cramped. There's no way that I know of to fix that without mods. If you do know a way, please let me know and I'll update the guide.

I wanted to do this vanilla so I could get achievements, I'm also not really a fan of playing with a tonne of mods.

If you want me to make a guide on how to use mods with Fallout on Linux, I can give it a shot.

There are a few people I want to thank, without them telling us how to fix it on Windows I could not have made this guide for Linux.

Thanks to Nachtstahl from this discussion for linking a video they found on YouTube which shows you how to set 21:9 resolutions.

And thanks to AOD Gaming for publishing the YouTube video: Fallout 4 : How to Ultra Wide 21:9 and Custom Resolutions (PC)

Any issues please let me know and I'll try and help you though them, happy vault hunting!