I've got a mid 2012 15" MBP running 10.10.5.
I've also got access to a mid 2012 13" MBP running 10.12.6.
I'm trying to connect both (separately one at a time for testing) to this LG ultra wide display which is a 21:9, 2560*1080.
I chose this monitor as its the widest these 2 MBPs can support according to their tech docs here:
Both of those docs state:
Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
When I connect the 15" MBP to the display it works fine and I can select 2560 x 1080. (see screenshot below)
But when I connect the 13" MBP the biggest size is can set is 1280 x 800, even if I 'option key' click the scaled button. (see screenshot below) Any ideas why I can't access the full set of screen dimensions on the 13" MBP ?