This is probably not something that will bother any other users! But if anyone can reproduce or correct any misunderstanding of mine then I'd be interested please.
As I understand it, the arrows here should be active (light grey). The right arrow ('Selects the next object') should only be in this inactive, medium grey state when the very last object on the track is selected. Because there will then be no 'next object' to select. Similarly for the left arrow ('Selects the previous object'), only if it is the very first in the track.
But I find that mostly (it's annoyingly inconsistent!) whenever the object is video, the arrows are inactive. And that's regardless of how I select the video object (by clicking, by Shift+X, or with the arrows).
Plays havoc with my macros!