I played around with some applications, and they all behave differently, so I don't think there is a standard, but NS 4 and earlier had it pop up on mouse down, this is nice because you can do things like this: mouse down, select back, mouse up mouse down, select save image, mouse up the way it is now, you have to do this: mouse down, mouse up, select back, mouse down, mouse up Also, when it comes up on mouse up, it makes the UI feel "laggy." Even though it really isn't laggy, it kinda feels that way with the popup menus for this reason. There's really no reason to wait for mouse up is there?
I tested this on Linux build 2000-10-10-15. Results: The context menu does pop up "onMouseDown", but menu items cannot be selected with click-point-release" as in other X applications I'm using - you'll have to use the "click-release-point-click-release" way. There might be two different bugs here, one of which seems to be Windows-only. Investigating further.
This one is a dup of bug 49844, and the other problem I noticed is bug 16766. *** This bug has been marked as a duplicate of 49844 ***