Updates the view rect to match the current tree value.

def _updateViewRect( self ):
        """
        Updates the view rect to match the current tree value.
        """
        if not self.updatesEnabled():
            return
        
        header_h    = self._cellHeight * 2
        rect        = self.uiGanttVIEW.scene().sceneRect()
        sbar_max    = self.uiGanttTREE.verticalScrollBar().maximum()
        sbar_max   += self.uiGanttTREE.viewport().height() + header_h
        widget_max  = self.uiGanttVIEW.height()
        widget_max -= (self.uiGanttVIEW.horizontalScrollBar().height() + 10)
        
        rect.setHeight(max(widget_max, sbar_max))
        self.uiGanttVIEW.scene().setSceneRect(rect)