Q: 34" LG Ultrawide and MBPR 15"Sierra resolution issue

Having the same problem, but only connected at home desk today...lost some time with cables checks before wondering about Sierra update .... Let's hope a fix's going to come fast.

I could make my LG ultra wide (2560 X 1080) work with SwichResX.  But I needed to:

1 - Disable System Integrity Protection (instruction on their site or anywhere else - I don't know if I can paste in here)

2 - Create a custom resolution, based on the 1080P60 detected.  Just used the same parameters, changing only the first parameter of the resolution (change 1920 to 2560).  Then save and reboot.

I could make my LG ultrawide monitor work properly with SwitchResX, but after 2 more steps:

1 - I disabled System Integrity Protection before installing SwitchResX (http://www.madrau.com/support/support/srx_1011.html)

2 - I created a custom resolution based on the 1080P60 (automatically detected), just changing the first resolution parameter from 1920 to 2560 (don't forget to save and reboot the computer after creating the custom resolution - check the status column next to the resolution created).  Those are the parameters of the custom resolution:

I've been searching and trying fix this problem for three days, after upgrading for Sierra.  I hope it helps!!

Zanatta's solution works perfectly for me.

Everyone have this issue should try it out:

1. Reboot and hold Cmd+R, go to Utilities>Terminal

2. Type: csrutil disable, hit Return then reboot

3. Install SwitchResX, select your monitor in left panel > Custom Resolutions

4. Hit '+' button at bottom, change 1920 to 2560 > Save > Apply Now button > Reboot

5. You'll find 2560 * 1080 resolution in system preference. Done.

Oh and don't forget to turn on SIP:

Reboot and hold Cmd+R, go to Utilities > Terminal > csrutil enable > Return and reboot

I have an LG UltraWide monitor as well, 2560x1080 resolution. Stumbled upon this thread and was frustrated for a while with the same problem. The public beta that Apple released yesterday (10.12.1 beta 2) includes a fix for this problem (I am using my UltraWide at full resolution as we speak). If you're really anxious, I would go online and register for the public beta so you can start using the pixels you paid for! Otherwise, I think 10.12.1 will fix it when it is officially released.

Just updated to "OS Sierra beta" latest version and it fixed the problem.

Can confirm. Updated to the newest beta and my Acer Predator now defaults to 3440x1440! Hooray.

Same problem.

I have the same issue with my LG Ultrawide 34 Inch Monitor with my Mac Book Pro 15.4 inch Retina Display 2014...After upgrading to Sierra....The resolution looks awful!! Not sure how to address this?

I have the same issue, fix it

Same issue here.  UGH!  Any ideas on how to resolve?

As suggested, I have updated Sierra with Beta edition 10.12.1. This has fixed the issue but I am on Public Beta.

Updating to Sierra beta solved the issue for LG Ultrawide 34

Same issue for me with 34" UltraWide after updating.  Read on another thread that upgrading to Sierra Beta solved the issue.  This worked for me.