To use pop-up menus in the threads view, select a thread from the list
  or position your mouse pointer in the threads view, and press MB3.

  A pop-up menu with the following menu items appears:

  Abort        Requests that the selected thread be terminated
               at the next allowed opportunity.  The exact effect
               depends on the current event facility (which is
               language dependent).  For Ada tasks, this is
               equivalent to executing an abort statement.

  Activate     Makes the selected thread the active thread.

  Hold         Places the selected thread on hold.

  Nohold       Releases the selected thread on hold.

  Visible      Makes the selected thread the visible thread.