It doesn't fire in Safari 5 and WebKit ToT. It does in all other browsers I tested (Chrome 6.0.472.25, Firefox 4 beta 2, and Opera 10.60).
Created attachment 63998 [details] Reduction
It seems to be Mac OS X related issue. WebKit on Windows shows a context menu then the mouse button is released (i.e. onmouseup). On Mac OS X it shows right after the mouse button is down (i.e. onmousedown). Didn't test it on other platforms.
I am unable to reproduce this bug in Safari 15.6 on macOS 12.5 and upon right click, "mousemouse" event fire off right after contextmenu rather than waiting for button to be released, which was this bug about. Since, the event is fired right away and it is matching with all browsers (Chrome Canary 106 and Firefox 105), I think this bug can be marked as "RESOLVED CONFIGURATION CHANGED". Thanks!