THIS MOD IS NO LONGER MAINTAINED, USE TruBy9 INSTEAD!!

Ultra Widescreen Patches

• This mod patches the Fallout 4 Interface for 21:9 UW-UXGA resolutions.
  
  
• This mod is completely standalone and DOES NOT require Flawless Widescreen to function.
  
  
• This mod properly converts load screens, scopes, the Power Armor Rainbox and all the 3d icons you see in menus to a 21:9 aspect ratio.
  
  

Simply install this mod using your favorite mod manager.
I use ModOrganizer 2, but NMM works as well (if you feel like losing your mind managing billions of loose files in the same folder).

Don't forget to edit your Fallout4Prefs.ini file, under [Display] and add your 21:9 resolution settings:

iSize W=2560 ; or whatever 21:9 resolution you use
iSize H=1080
bTopMostWindow=0
bMaximizeWindow=0
bFull Screen=0
bBorderless=1

Update from 2.0

In 2.1 I switched from the .bsa archive as it was causing some issues. Remove the archive and the ini edits related to it (what you see in red down here), and install the new version using your favorite mod manager.

sResourceStartUpArchiveList=Fallout4 - Startup.ba2, Fallout4 - Shaders.ba2, Fallout4 - Interface.ba2, uw-uxga.bsa
SResourceArchiveList=Fallout4 - Voices.ba2, Fallout4 - Meshes.ba2, Fallout4 - MeshesExtra.ba2, Fallout4 - Misc.ba2, Fallout4 - Sounds.ba2, Fallout4 - Materials.ba2, uw-uxga.bsa

Workshop menu Icons

The workshop icons are still hardcoded to 16:9. Until properly fixed you can use the provided ini setting provided with the fomod installer.
Fixing those icons is quite a pain, as it involves scaling and repositioning a LOT of elements in the nif file.

Power Armor HUD

While not hardcoded to 16:9, the HUD of the power armor is not wide enough to cover the full 21:9 screen area. Until a proper fix is implemented, you can use the ini settings provided with the fomod installer.

Properly fixing this means editing the .nif model and adding missing metal pieces to the left & right of the power armor screen. It's not super easy as it involves redoing the UVW mapping/textures as well.

Random Questions

Why not use Widescreen fix you ask? Don't get me wrong Widescreen fix is great, and it allowed me to enjoy the game so far. But not all files are patched and the text is way too small for my tastes. Also, it requires Flawless Widescreen, something I want to avoid having to run constantly.

What about PA RainBox HUD Fix? Doesn't it fix the Power Armor Rainbox HUD? Yes, yes it does. But my fix is a little different: I keep all the parts of the original interface, only rescaled to 21:9. I even rearranged the rain particles so they don't just cover the 16:9 part of the screen. I'm pretty proud of my rainbox fix.

How?

Most patches are SWF patches, which is a very simple edit in most cases. The article linked to this mod describe how I edited those files.
Some patches are nif patches. The game uses .nif files to draw some interface elements, and those nifs use a hardcoded 16:9 aspect ratio. These nifs control the the semi-transparent green backgrounds, the scope menu, the Workshop menu, the Power Armor rainbox, the loading screen, main menu screens, and so forth. So I went ahead and converted these nifs to 21:9 using 3ds max and nifskope.
You can see the results in the screenshots above.
The loading / esc menus also needed a rescaled texture.

Kudos

Widescreen fix - The original widescreen fix.
PA RainBox HUD Fix - The original Power Armor Rainbox fix.