Have you been able to confirm you are using the very latest version of JUCE?
Haven’t had a chance to update to even test, but I plan to. Busy at the moment getting a new release out the door. The bottom line, though, is that no other host on the Mac has this issue, and the lead developer of JUCE himself has also stated in no uncertain terms that “… it sounds like something that’d be impossible to fix.”
My hypothesis (and I doubt that I’m wrong about it) is that WaveLab on the Mac has some specific way of handling mouse focus due to the tabbed interface, which may have unintended side effects such as this. In any case, I will conduct my own investigation as soon as time permits.
FWIW. I may have just had a case where the mouse cursor was stuck on a montage tab, so it may not even be 100% plugin related. Was not holding the mouse button down but it continued to drag a montage tab to a new location.
Though I wasn’t using it in the moment, I did just add this to my setup:
http://nobcontrol.com
So it remains to be seen if it has any influence on the mouse operation or if what I just saw is related to the plugin issue.