I've been playing with some MIDI gadgets over the past few days. I enabled MIDI output to send feedback to my Qu-Neo controller. Aside from hours of MIDI routing etc frustration, I noticed that if you use the mouse to drag a layer's opacity slider, if you move the mouse quickly from within the slider range to outside it (above or below), although the slider moves to the extremity (top or bottom), the MIDI output stops updating at - I'm guessing - the last place the mouse was within the range (before the next screen refresh when the mouse is out of the range).
I haven't looked to see if this happens with any other mouse-draggable controls.