Java Swing JScrollPane not scrolling when shift held down and using mouse wheel

Question

I have a simple Java Swing program defined below:

import javax.swing.*;
import java.awt.*;

public class Test implements Runnable {

    public static void main(String[] args) {
        Test main = new Test();
        SwingUtilities.invokeLater(main);
    }
    
    @Override
    public void run() {

        // Application window.
        JFrame mainFrame = new JFrame();

        // Set up window.
        mainFrame.setPreferredSize(new Dimension(600,700));
        mainFrame.setFocusable(true);
        mainFrame.requestFocus();

        JScrollPane scrollPane = new JScrollPane();
        scrollPane.setPreferredSize(new Dimension(600,700));

        JPanel scrollPanel = new JPanel();
        scrollPanel.setLayout(new GridLayout(0,1));

        // Add buttons.
        for (int i = 0; i < 40; i ++) {
            JButton button = new JButton("Button " + i);
            button.setPreferredSize(new Dimension(600,100));
            scrollPanel.add(button);
        }

        scrollPane.setViewportView(scrollPanel);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

        JPanel mainPanel = new JPanel();
        mainPanel.add(scrollPane);

        // Fill up window.
        mainFrame.getContentPane().setLayout(new BorderLayout());
        mainFrame.getContentPane().add(mainPanel, BorderLayout.CENTER);
        SwingUtilities.updateComponentTreeUI(mainFrame);

        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.pack();
        mainFrame.setVisible(true);

    }
}

The program consists of a simple JScrollPane with multiple buttons inside of it. Only vertical scrolling is enabled. It works fine.

However, the problem is, when I am holding down the 'shift' key, vertical scrolling does not work when I am using the mouse wheel to scroll. Vertical scrolling only works when I drag the scrollbar or let go of the 'shift' key.

Usually, in a JScrollPane, when the 'shift' key is held down, and the mouse wheel is used, it scrolls horizontally instead of vertically. However, I have disabled horizontal scrolling.

How would I go about enabling vertical scrolling using the mouse wheel if the user is holding down 'shift'?

Answer 1

I have figured out a cleaner and simpler answer to my question. Here is the code:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class CustomScrollPane extends JScrollPane {

    public CustomScrollPane(Component component) {
        super(component);
        initialize();
    }

    private void initialize() {

        // Set up the scroll bar.
        setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

        // Disable scrolling so we can do it manually.
        setWheelScrollingEnabled(false);

        addMouseWheelListener(new MouseWheelListener() {
            @Override
            public void mouseWheelMoved(MouseWheelEvent e) {
                // Manually scroll.
                getVerticalScrollBar().setValue(getVerticalScrollBar().getValue() + (e.getWheelRotation()));
            }
        });
    }
}

The solution involves disabling JScrollPane from scrolling. It detects when the mousewheel is being rotated, and manually scrolls. This way, it will scroll when the mousewheel is rotated; it does not matter if 'shift' is being held down or not. The only drawback that I can see is that getVerticalScrollBar().setUnitIncrement() will not change the scrolling speed. The scrolling speed has to be changed manually, perhaps with a constant multiplier.

Answer 2

I don't think there is a simple way to do this because normally you would only need the shift scroll when scrolling in more than one direction.

You could try adding a key listener to your JFrame and setting the wheel on your mouse to scroll vertically whenever the shift key is pressed. I tried it with this code and it worked just fine:

frame.addKeyListener(new KeyListener() {

        @Override
        public void keyPressed(KeyEvent arg0) {
            // TODO Auto-generated method stub
            if(arg0.isShiftDown()) {
                frame.addMouseWheelListener(new MouseWheelListener() {

                    @Override
                    public void mouseWheelMoved(MouseWheelEvent arg0) {
                        // TODO Auto-generated method stub
                        pane.getVerticalScrollBar().setValue(pane.getVerticalScrollBar().getValue()+arg0.getWheelRotation());
                    }

                });
            }
            if(!arg0.isShiftDown()) {
                frame.removeMouseWheelListener(frame.getMouseWheelListeners()[0]);
            }
        }
}