title: grab scroll in scrollview not working as expected
body: <p>I'm trying to make a scrollview scroll by clicking and dragging on it. It does scroll but not nicely, it looks like it's rapidly alternating between scrolling the right way and scrolling in the opposite direction than it's supposed to. Anyway's here's what I've got so far
  (this is in a custom mouse adapter):</p>
  
  <pre><code>point p;
  JViewport viewport;
  @Override
  public void mousePressed(MouseEvent e) 
  {
          p.setLocation(e.getX(),e.getY());
      }
  @Override
  public void mouseDragged(MouseEvent e)
  {
          Point ep = new Point();
          ep.setLocation(e.getX(),e.getY());
          double changex =  e.getX() - p.getX();
          double changey = e.getY() - p.getY();
          Point q = viewport.getViewPosition();
          q.setLocation(q.getX()-changex, q.getY()-changey);
          p.setLocation(ep);
          viewport().setViewPosition(q);
      }
  </code></pre>