When I've set the mixer panel layout to "Extended Mixer Sidebar" and scroll through the mixer with the mousewheel, it happens that the cursor lands on a list of sends which - using this particular layout - reaches to the bottom of the panel, so it suddenly stops scrolling through the mixer.
Maybe you could just add an extra checkbox under Preferences -> Mouse for ignoring the mousewheel on those lists. Or mousewheel actions on this layout are disabled by default. The lists are long enough I guess.
Dunno if this really is a bug and post-worthy...
But man... besides that the "Extended Mixer Sidbar"-layout rocks!