Running nightly 18.0a1 (2012-09-07) on OSX 10.7.4 with the browser window on an external monitor. Sometimes the context menu will repaint repeatedly, slightly lower and to the right of where it was until its in the lower right hand corner. I took a video of the behavior with my phone, will attach. Seems to be location based (repeatedly opening a context menu on the same spot has the same result, arbitrarily right-clicking spawns some that move and some that don't). Can reproduce in multiple tabs loaded with different pages.
attachment failed, so I put it on youtube: http://youtu.be/e44s5nGKy8o
can reproduce in 18.0a1 nightlies for 09-10 and 09-11 (latest) also