@see "Mouse Dragging ViewPort Scroll - Java Swing Tips" comment: http://java-swing-tips.blogspot.jp/2009/03/mouse-dragging-viewport-scroll.html

DragScrollLayerTest.java

import java.awt.*;
import java.awt.event.*;
import java.beans.PropertyChangeEvent;
import javax.swing.*;
import javax.swing.plaf.LayerUI;

class DragScrollLayerTest {
    public JComponent makeUI() {
        JTabbedPane tab1 = new JTabbedPane();
        tab1.addTab("aaa", new JLabel("11111111111"));
        tab1.addTab("bbb", new JLabel("2222222222"));

        JTabbedPane tab2 = new JTabbedPane();
        tab2.addTab("ccccc", new JLabel("3333"));
        tab2.addTab("ddddd", new JLabel("444444444444"));

        Box box = Box.createVerticalBox();
        box.add(new JLabel("aaaaaaaaaaaaaaaaaaaaaa"));
        box.add(Box.createVerticalStrut(5));
        box.add(tab1);
        box.add(Box.createVerticalStrut(5));
        box.add(new JCheckBox("bbbbbbbbbbbb"));
        box.add(Box.createVerticalStrut(5));
        box.add(tab2);
        box.add(Box.createVerticalStrut(5));
        box.add(new JButton("ccccc"));
        box.add(Box.createVerticalStrut(5));
        box.add(new JScrollPane(new JTree()));
        box.add(Box.createVerticalStrut(5));
        box.add(new JLabel("ddddddddddddddd"));
        box.add(Box.createVerticalGlue());

        box.setBorder(BorderFactory.createTitledBorder("Title"));
        JScrollPane scroll = new JScrollPane(box);
        scroll.getVerticalScrollBar().setUnitIncrement(25);
        return new JLayer<JScrollPane>(scroll, new DragScrollLayerUI());
    }
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override public void run() {
                createAndShowGUI();
            }
        });
    }
    public static void createAndShowGUI() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.getContentPane().add(new DragScrollLayerTest().makeUI());
        frame.setSize(320, 240);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}
class DragScrollLayerUI extends LayerUI<JScrollPane> {
    private final Point pp = new Point();
    @Override public void installUI(JComponent c) {
        super.installUI(c);
        JLayer jlayer = (JLayer)c;
        jlayer.setLayerEventMask(
            AWTEvent.MOUSE_EVENT_MASK | AWTEvent.MOUSE_MOTION_EVENT_MASK |
            AWTEvent.MOUSE_WHEEL_EVENT_MASK | AWTEvent.KEY_EVENT_MASK);
    }
    @Override public void uninstallUI(JComponent c) {
        JLayer jlayer = (JLayer)c;
        jlayer.setLayerEventMask(0);
        super.uninstallUI(c);
    }
    @Override protected void processMouseEvent(MouseEvent e, JLayer<? extends JScrollPane> l) {
        if(e.getID()==MouseEvent.MOUSE_PRESSED) {
            JViewport vport = l.getView().getViewport();
            Point cp = SwingUtilities.convertPoint(e.getComponent(), e.getPoint(), vport);
            pp.setLocation(cp);
        }
    }
    @Override protected void processMouseMotionEvent(MouseEvent e, JLayer<? extends JScrollPane> l) {
        if(e.getID()==MouseEvent.MOUSE_DRAGGED) {
            JViewport vport = l.getView().getViewport();
            JComponent cmp = (JComponent)vport.getView();
            Point cp = SwingUtilities.convertPoint(e.getComponent(), e.getPoint(), vport);
            Point vp = vport.getViewPosition();
            vp.translate(pp.x-cp.x, pp.y-cp.y);
            cmp.scrollRectToVisible(new Rectangle(vp, vport.getSize()));
            pp.setLocation(cp);
        }
    }
}