http://ateraimemo.com/Swing/TreeDragScroll.html#comment-3070791028

KineticScrollingViewPositionTest.java

//package example;
//-*- mode:java; encoding:utf-8 -*-
// vim:set fileencoding=utf-8:
//@homepage@
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;
import javax.swing.Timer;
import javax.swing.tree.*;

public final class KineticScrollingViewPositionTest {
  private JComponent makeUI() {
    JTree tree = new JTree();
    tree.setOpaque(false);

    JScrollPane scroll = new JScrollPane(tree);
    JViewport viewport = new JViewport() {
      private boolean validateIsAdjusting;
      @Override public void revalidate() {
        if (validateIsAdjusting) {
          return;
        }
        super.revalidate();
      }
      @Override public void setViewPosition(Point p) {
        validateIsAdjusting = true;
        super.setViewPosition(p);
        validateIsAdjusting = false;
      }
    };
    viewport.setView(tree);
    //viewport.setScrollMode(JViewport.BACKINGSTORE_SCROLL_MODE);
    viewport.setBackground(tree.getBackground());
    scroll.setViewport(viewport);

    MouseAdapter ma1 = new KineticScrollingListener2(tree);
    viewport.addMouseMotionListener(ma1);
    viewport.addMouseListener(ma1);

    MouseAdapter dispatcher = new MouseAdapter() {
      private void dispatchEvent(MouseEvent e) {
        JComponent c = (JComponent) e.getComponent();
        JComponent p = (JComponent) e.getComponent().getParent();
        p.dispatchEvent(SwingUtilities.convertMouseEvent(c, e, p));
      }
      @Override public void mouseDragged(MouseEvent e)  {
        dispatchEvent(e);
      }
      @Override public void mouseClicked(MouseEvent e)  {
        dispatchEvent(e);
      }
      @Override public void mouseEntered(MouseEvent e)  {
        dispatchEvent(e);
      }
      @Override public void mouseExited(MouseEvent e)   {
        dispatchEvent(e);
      }
      @Override public void mousePressed(MouseEvent e)  {
        dispatchEvent(e);
      }
      @Override public void mouseReleased(MouseEvent e) {
        dispatchEvent(e);
      }
    };
    tree.addMouseMotionListener(dispatcher);
    tree.addMouseListener(dispatcher);

    return scroll;
  }
  public static void main(String... args) {
    EventQueue.invokeLater(() -> {
      createAndShowGUI();
    });
  }
  public static void createAndShowGUI() {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (ClassNotFoundException | InstantiationException
           | IllegalAccessException | UnsupportedLookAndFeelException ex) {
      ex.printStackTrace();
    }
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.getContentPane().add(new KineticScrollingViewPositionTest().makeUI());
    frame.setSize(320, 240);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}

class KineticScrollingListener2 extends MouseAdapter implements HierarchyListener {
  private static final int SPEED = 4;
  private static final int DELAY = 10;
  private static final double D = .8;
  private final JComponent label;
  private final Point startPt = new Point();
  private final Point delta   = new Point();
  private final Cursor dc;
  private final Cursor hc = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR);
  private final Timer inside = new Timer(DELAY, new ActionListener() {
    @Override public void actionPerformed(ActionEvent e) {
      JViewport vport = (JViewport) SwingUtilities.getUnwrappedParent(label);
      Point vp = vport.getViewPosition();
      //System.out.format("s: %s, %s%n", delta, vp);
      vp.translate(-delta.x, -delta.y);
      vport.setViewPosition(vp);
      if (Math.abs(delta.x) > 0 || Math.abs(delta.y) > 0) {
        delta.setLocation((int)(delta.x * D), (int)(delta.y * D));
        //Outside
        if (vp.x < 0 || vp.x + vport.getWidth() - label.getWidth() > 0) {
          delta.x = (int)(delta.x * D);
        }
        if (vp.y < 0 || vp.y + vport.getHeight() - label.getHeight() > 0) {
          delta.y = (int)(delta.y * D);
        }
      } else {
        //inside.stop();
        ((Timer) e.getSource()).stop();
        if (!isInside(vport, label)) {
          outside.start();
        }
      }
    }
  });
  private final Timer outside = new Timer(DELAY, new ActionListener() {
    @Override public void actionPerformed(ActionEvent e) {
      JViewport vport = (JViewport) SwingUtilities.getUnwrappedParent(label);
      Point vp = vport.getViewPosition();
      //System.out.format("r: %s%n", vp);
      if (vp.x < 0) {
        vp.x = (int)(vp.x * D);
      }
      if (vp.y < 0) {
        vp.y = (int)(vp.y * D);
      }
      if (vp.x + vport.getWidth() - label.getWidth() > 0) {
        vp.x = (int)(vp.x - (vp.x + vport.getWidth() - label.getWidth()) * (1d - D));
      }
      if (vp.y + vport.getHeight() > label.getHeight()) {
        vp.y = (int)(vp.y - (vp.y + vport.getHeight() - label.getHeight()) * (1d - D));
      }
      vport.setViewPosition(vp);
      if (isInside(vport, label)) {
        //outside.stop();
        ((Timer) e.getSource()).stop();
      }
    }
  });

  private static boolean isInside(JViewport vport, JComponent comp) {
    Point vp = vport.getViewPosition();
    return vp.x >= 0 && vp.x + vport.getWidth()  - comp.getWidth()  <= 0
           && vp.y >= 0 && vp.y + vport.getHeight() - comp.getHeight() <= 0;
  }
  protected KineticScrollingListener2(JComponent comp) {
    super();
    this.label = comp;
    this.dc = comp.getCursor();
  }
  @Override public void mousePressed(MouseEvent e) {
    e.getComponent().setCursor(hc);
    startPt.setLocation(e.getPoint());
    inside.stop();
    outside.stop();
  }
  @Override public void mouseDragged(MouseEvent e) {
    Point pt = e.getPoint();
    JViewport vport = (JViewport) SwingUtilities.getUnwrappedParent(label);
    Point vp = vport.getViewPosition();
    vp.translate(startPt.x - pt.x, startPt.y - pt.y);
    vport.setViewPosition(vp);
    delta.setLocation(SPEED * (pt.x - startPt.x), SPEED * (pt.y - startPt.y));
    startPt.setLocation(pt);
  }
  @Override public void mouseReleased(MouseEvent e) {
    e.getComponent().setCursor(dc);
    JViewport vport = (JViewport) SwingUtilities.getUnwrappedParent(label);
    if (isInside(vport, label)) {
      inside.start();
    } else {
      outside.start();
    }
  }
  @Override public void hierarchyChanged(HierarchyEvent e) {
    if ((e.getChangeFlags() & HierarchyEvent.DISPLAYABILITY_CHANGED) != 0 && !e.getComponent().isDisplayable()) {
      inside.stop();
      outside.stop();
    }
  }
}