Using the sample code from bug #4108907, run the program and do the following:

Click on the menu to pull it down. Click in one of the buttons. Menu will unpost, button will "go down", then (as you release the mouse button) the  button will "go up", and it will aquire a highlight (i.e., it is selected). Now, click on the menu again. The menu will pull down, and the highlighted button will unhighlight (as it should). Now (while the menu is down) click in the *other* button. The menu will unpost, the button will go down, then up, then aquire a highlight, just like the other button did before. So far so good. Now, for the punch line, click on the menu to pull it down again. The menu will pull down, but the highlighted button will remain highlighted.

Via a sequence of events which I can't completely characterize (but which I believe is deterministic and reproducible), it's possible to have both the buttons highlighted, and also have the menu down. Major confusion.