Alt+Scroll is in Xubuntu overloaded to have two behaviors: alter window transparency when mouse is positioned on title bar, or zoom otherwise.
When I position the mouse cursor on a window title bar and scroll down, the window gets less transparent. As expected.
If I scroll upwards, hoping to restore opacity, the view zooms in. If I zoom in to maximum magnification, the opacity-control resumes, and I can restore opacity to my windows.
I would ideally like to be able to do both, but I don't want to zoom in to restore opacity to my windows. Is that possible? If so, how?
SOLVED: Turns out it is the horizontal scroll on window decorator that causes the transparency change. I had foolishly turned that on and scrolled with a touchpad, inevitably scolling hotizontally on accident.