Comment 3 for bug 1798453

I think I can see this: it happens if the mouse moves during right-click:

* Move mouse to canvas
* Right click and hold: menu appears at click location
* Move mouse, menu remains in place during move
* Release: menu disappears. If the mouse was over a menu item, it is activated, if not, nothing happens.

This is probably easier to do accidentally on sensitive mouses, and even a tiny or brief move in step 3 is enough.