code: protected void checkAutoscroll (MouseEvent exitEvent)
      {
          Component comp = exitEvent.getComponent();
          Point p = exitEvent.getPoint();
          try {
              Point scr = comp.getLocationOnScreen();
              p.translate(scr.x, scr.y);
          } catch (IllegalComponentStateException icse) {
              // the component is no longer on screen. Deal.
              return;
          }
  
          Component parent;
          DropTarget target;
          while (true) {
              target = _droppers.get(comp);
              if (target instanceof AutoscrollingDropTarget) {
                  AutoscrollingDropTarget adt = (AutoscrollingDropTarget) target;
                  JComponent jc = (JComponent) comp;
  
                  Rectangle r = getRectOnScreen(jc);
                  // make sure we're actually out of the autoscrolling component
                  if (!r.contains(p)) {
                      // start autoscrolling.
                      _scrollComp = jc;
                      _scrollDim = adt.getAutoscrollBorders();
                      _scrollPoint = p;
                      _scrollTimer.start();
                      return;
                  }
              }
  
              parent = comp.getParent();
              if (parent == null) {
                  return;
              }
              comp = parent;
          }
      }
docs: Check to see if we want to enter autoscrolling mode.