code: function(ev) {
              // Ignore right clicks
              if (ev.which !== Garnish.PRIMARY_CLICK) {
                  return;
              }
  
              // Ignore if we already have a target
              if (this.$targetItem) {
                  return;
              }
  
              // Ignore if they didn't actually click on the handle
              var $target = $(ev.target),
                  $handle = this._getItemHandle(ev.currentTarget);
  
              if (!$target.is($handle) && !$target.closest($handle).length) {
                  return;
              }
  
              // Make sure the target isn't a button (unless the button is the handle)
              if (ev.currentTarget !== ev.target && this.settings.ignoreHandleSelector) {
                  if (
                      $target.is(this.settings.ignoreHandleSelector) ||
                      $target.closest(this.settings.ignoreHandleSelector).length
                  ) {
                      return;
                  }
              }
  
              ev.preventDefault();
  
              // Make sure that dragging is allowed right now
              if (!this.allowDragging()) {
                  return;
              }
  
              // Capture the target
              this.$targetItem = $(ev.currentTarget);
  
              // Capture the current mouse position
              this.mousedownX = this.mouseX = ev.pageX;
              this.mousedownY = this.mouseY = ev.pageY;
  
              // Capture the difference between the mouse position and the target item's offset
              var offset = this.$targetItem.offset();
              this.mouseOffsetX = ev.pageX - offset.left;
              this.mouseOffsetY = ev.pageY - offset.top;
  
              // Listen for mousemove, mouseup
              this.addListener(Garnish.$doc, 'mousemove', '_handleMouseMove');
              this.addListener(Garnish.$doc, 'mouseup', '_handleMouseUp');
          }
docs: Private methods --------------------------------------------------------------------- 
  Handle Mouse Down