It happens in WaveLab 12.0.51 on Windows 10 64-bit ( both updated ), when opening a Project with a Montage File Group, using three or more Tracks—where two of them are initially visible and vertical scrollbar to the right doesn’t work, since it’s fully stretched across whole vertical scrollbar space.
To fix the scrollbar, I open the Montage window by double-clicking on it, so it opens up “Full-Screen” and then again, to restore it’s initial view. After that maneuver, vertical scrollbar resizes it self to a correct size, and starts working, so I can see the rest of the tracks—scrolling up and down.
Update: Vertical scrollbar works as intended today ( a day after report ), so it doesn’t happen always apparently. I’ve noticed that more than once, but didn’t look closer—which I will do from now on, and report here, if I find a consistent behavior, so anyone can test it and look deeper into the issue.