Why Does My Embedded Canvas in NetBeans' JSplitPane Disappear When Resizing?

Resizing Issues with Embedded Canvas in NetBeans GUI Editor

In building an application with a JSplitPane, containing a Canvas within a JScrollPane as the top component, users may encounter resizing problems. When pushing the divider upwards, the divider tends to disappear beneath the Canvas.

Various adjustments to the preferred, minimum, and maximum sizes of the JScrollPane and Canvas have proven ineffective in resolving this issue. Here's a closer look at the pertinent code generated by NetBeans:

jSplitPane1 = new javax.swing.JSplitPane();
jScrollPane1 = new javax.swing.JScrollPane();
canvas1 = new java.awt.Canvas();
jTextField1 = new javax.swing.JTextField();

jSplitPane1.setDividerLocation(300);
jSplitPane1.setOrientation(javax.swing.JSplitPane.VERTICAL_SPLIT);

jScrollPane1.setViewportView(canvas1);

jSplitPane1.setTopComponent(jScrollPane1);

jTextField1.setText("jTextField1");
jSplitPane1.setRightComponent(jTextField1);

The culprit lies in the use of setPreferredSize() for the Canvas component. By setting a fixed size, the Canvas becomes inflexible and restricts the resizing capabilities of the surrounding JScrollPane.

A solution to this issue is to remove the fixed size and let the Canvas dynamically calculate its preferred size. This can be achieved by using pack() on the enclosing window to accommodate the dynamic size changes.

Here's an example that solves the resizing problem:

import draw.GraphPanel;
import java.awt.EventQueue;
import java.awt.GridLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;

/**
 * @see https://stackoverflow.com/q/11942961/230513
 */
public class SplitGraph extends JPanel {

    public SplitGraph() {
        super(new GridLayout());
        JSplitPane split = new JSplitPane(JSplitPane.VERTICAL_SPLIT);
        GraphPanel graphPanel = new GraphPanel();
        split.setTopComponent(new JScrollPane(graphPanel));
        split.setBottomComponent(graphPanel.getControlPanel());
        this.add(split);
    }

    private void display() {
        JFrame f = new JFrame("SplitGraph");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(this);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new SplitGraph().display();
            }
        });
    }
}

In this example, the preferred size of the GraphPanel is dynamically calculated and adjusts appropriately when the divider is moved. This allows for a seamless resizing experience without any disappearing divider issues.

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