In some cases (could not reproduce, but had it already twice in the released 3.0 Creator that uses 5.2.0) the contextual menu is always triggered.
I attach a movie showing the behaviour
in it is just scroll around and the contextual menu is constantly triggered.
I even trigger some menu choice in it, always just scrolling, (I immediately undo it with cmd-Z).
Then I show that other applications (terminal) menu do work normally (and I trigger a contextual menu with right click).
The frequency of auto triggering was very much slower when recording the screencast than during normal execution (in which the menu immediately follows the mouse cursor.
This might be due to the menu being key, and thus have been fixed by https://codereview.qt-project.org/#change,75440
I did try to get some stack traces, but I got just
CFRunLoopSourceSignal > msg send > [NSEvent otherEventWithType:location:modifierFlags:timestamp:windowNumber:context:subtype:data1:data2:]