code: function show() {
  
                popupTimeout = null;
  
                // If there is a pending remove transition, we must cancel it, lest the
                // tooltip be mysteriously removed.
                if ( transitionTimeout ) {
                  $timeout.cancel( transitionTimeout );
                  transitionTimeout = null;
                }
  
                // Don't show empty tooltips.
                if ( !(options.useContentExp ? ttScope.contentExp() : ttScope.content) ) {
                  return angular.noop;
                }
  
                createTooltip();
  
                // Set the initial positioning.
                tooltip.css({ top: 0, left: 0, display: 'block' });
                ttScope.$digest();
  
                positionTooltip();
  
                // And show the tooltip.
                ttScope.isOpen = true;
                ttScope.$apply(); // digest required as $apply is not called
  
                // Return positioning function as promise callback for correct
                // positioning after draw.
                return positionTooltip;
              }
docs: Show the tooltip popup element.