I removed the redundant test here (http://hg.savannah.gnu.org/hgweb/octave/rev/565d724ecf50).

Marking as fixed and closing report.

I just noticed in the code:

  bool HoverToolButton::eventFilter (QObject *obj, QEvent *ev)
  {
    if (ev->type () == QEvent::HoverEnter)
      emit hovered_signal ();
    else if (ev->type () == QEvent::MouseButtonPress ||
             ev->type () == QEvent::MouseButtonPress)
      emit popup_shown_signal ();

    return QToolButton::eventFilter (obj, ev);
  }

has a redundant test for MouseButtonPress.  I can't recall what I was thinking at the time, but I'll look at this again at some point.  The hover tool button is the graph button that pops up a context menu.  Maybe I was thinking the second one should be "mouse double click" whatever expression is defined for that event.