I've built a Windows 8.1 PC with Intel HD 4600 graphics (i7 4765t) (driver version 10.18.10.3379), connected to a Dell U2412M monitor by DisplayPort.
The monitor's native resolution is 1920x1200, but I do not get this as an option in Windows' display settings. I heard that I should add a custom resolution using Intel's CustomModeApp.exe tool. I tried to do this but it appears to not have any way to apply the new resolution.
The new resolution I create (1920x1200 60Hz GTF) appears in the list on the right of the panel, but I see no way to make this become an option to activate in Windows? Restarting Windows does not make it appear in the available resolutions either.
How do I activate a 'custom resolution'?
I found the answer by accident.
Turns out the new build setup had somehow got a secondary monitor (which doesn't exist) and considered the display to be mirrored across both. So it was probably not allowing my native resolution because it wasn't supported by this phantom monitor that it thought was the primary one.
I set the display to extend rather than mirror and set my real monitor as the primary display and all is now well.