dialogs that cannot be moved that require a response pop up under menus
that  themselves cannot be deleted or moved.  The result is that you can
only respond to the dialog, or do anything else (as the dialogue inhibits
other actions until a response has been given), by guessing the correct
number of times to hit tab, and then pressing return.

I have found that this problem occurs frequently with the
middle-button-triggered  menu, but I think that I have found it to be the
case with the "normal" drop-down menu too.  I intend to update this report
when I find evidence of this.