Ha, my mouse scroll partially stopped - turns out it's a rotary encoder like you describe, some of the gaps get blocked with dust and the mouse stops scrolling, a bit, still scrolls but seems intermittent (like a software issue), was a couple of days before I opened it up as I was looking for a fault in logs and such.

Of course opening a mouse up and not breaking it is quite hard, they're made to break and be replaced. Current bugbear is Logitech keyboard stand pieces that appear only to be there to break and thus require keyboard replacement - again it seems a design with a penny's more of plastic would be almost invincible.