The LG 34um95 yosemite resolution bug is pretty annoying. I have an lg 34um95 screen – it runs at 3440×1440 – its a great screen, unless you have upgraded to OSX Yosemite.
This issue is now fixed on Yosemite 10.10.2 – if you are using a macbook air, then you can get 2560×1080 with your laptop open, and you can get the full 3440×1440 if you close your laptop and use only the LG screen
Just a quick update to confirm that this does indeed work on 10.10.1, though you have to do the whole process again after you run the update
I had a few looks about the forums, and found others with this issue too. eventually someone posted a potential workaround (that involves taking a feramebuffer driver form a mavericks install and modifying it and loading that during boot)
I tried this out, and it didn’t work (at first) so I had to come up with a few extra steps: here is the method I used.
1) download KextBeast
2)download the (pre-patched) mavericks kext file from this thread (thanks :D ), or follow these instructions and patch your own (remember that you need to patch the kext from Mavericks, not the one you have in Yosemite)
3)copy your existing AppleIntelFrameBufferCapri.kext somewhere safe
4)run this in a terminal
sudo nvram boot-args=kext-dev-mode=1
5)move (or copy) your now patched kext to your Desktop
6)run these commands in a terminal:
cd ~/Desktop sudo chmod -R 775 AppleIntelFramebufferCapri.kext sudo chown -R root:wheel AppleIntelFramebufferCapri.kext
7)run KextBeast which will install the patched kext from your desktop to where it needs to go
9)delete the patched kext from your desktop (or move it somewhere safe just incase)