Handle the event when a user clicks on one of the part buttons.
        
        :param      button | <QToolButton>

def handleButtonClick( self, button ):
        """
        Handle the event when a user clicks on one of the part buttons.
        
        :param      button | <QToolButton>
        """
        path            = button.property('path')
        is_completer    = button.property('is_completer')
        
        # popup a completion menu
        if ( unwrapVariant(is_completer) ):
            model = self.navigationModel()
            if ( not model ):
                return
            
            sep  = self.separator()
            path = nativestring(unwrapVariant(path))
            item = model.itemByPath(path, includeRoot = True)
            if ( not item ):
                return
            
            curr_path = nativestring(self.text()).strip(self.separator())
            curr_path = curr_path.replace(path, '').strip(self.separator())
            
            child_name = ''
            if ( curr_path ):
                child_name = curr_path.split(self.separator())[0]
            
            index = model.indexFromItem(item)
            
            self._completerTree.move(QCursor.pos())
            self._completerTree.setRootIndex(index)
            self._completerTree.verticalScrollBar().setValue(0)
            
            if ( child_name ):
                child_item = None
                for i in range(item.rowCount()):
                    child = item.child(i)
                    if ( child.text() == child_name ):
                        child_item = child
                        break
                
                if ( child_item ):
                    child_index = model.indexFromItem(child_item)
                    self._completerTree.setCurrentIndex(child_index)
                    self._completerTree.scrollTo(child_index)
            
            self._completerTree.show()
            self._completerTree.setUpdatesEnabled(True)
        else:
            self.setText(unwrapVariant(path))