jps wrote:IIRC nvidia drivers can sometimes intercept this key combination
Genius! Thank you!
I don't have nvidia drivers, but I do have an intel "Graphics Media Accelerator Driver For Mobile" program, which has a toggle to enable its hot keys for e.g., turning on & off the laptop & external displays. Those hotkeys don't include ctrl-alt-up/down, but the do all start with ctrl-alt and when I disable them sublime starts getting these keypresses & acts accordingly. So I'm fixed.
Thanks so much!