I'd say if you're just incidentally jostling around on horseback during a Combat Action, then you are free to do so in the same way you can incidentally wiggle and move on foot during combat. I would award no particular advantage to such movement, would just consider you routinely mobile with a possible
passive height advantage from horseback.
If engaged, a horse would not be able to gain its full movement rate, per the rule on p. 128. The horse would be constrained at the movement increments indicated on p. 126-127.
EDIT: (as your fussy GM, I would also cap your Combat skill at your Ride ability in mounted melee, as a measure of your control of your mount)...