But didn't Opera gestures always end on mouse-up? That makes it even more important that a context menu only opens on mouse-up, because otherwise you would potentially both open the menu and execute an action at the same time.

This is exactly why some people (including me) prefer this idiom -- right click down to make a context menu appear, nudge the mouse over a few pixels to select e.g. the back button, mouse-up and there you go.