ADDITIONAL SYSTEM INFORMATION :
Centos 7.5
3.10.0-862.14.4.el7.x86_64 #1 SMP Wed Sep 26 15:12:11 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)


A DESCRIPTION OF THE PROBLEM :
My JFrame wraps components whose total width exceeds the 1920 pixel width of my display. When my program launchs, I see that it sizes my JFrame to fit the monitor width. I was able to duplicate this problem using the FrameDemo .java program (https://docs.oracle.com/javase/tutorial/displayCode.html?code=https://docs.oracle.com/javase/tutorial/uiswing/examples/components/FrameDemoProject/src/components/FrameDemo.java).

 I modified it slightly to make the width of the emptyLabel object equal to 3000 pixels. Here is the exact code:

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

/* FrameDemo.java requires no other files. */
public class FrameDemo {
    /**
     * Create the GUI and show it. For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("FrameDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JLabel emptyLabel = new JLabel("blahblah");
        emptyLabel.setPreferredSize(new Dimension(3000, 100));
        frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

When I run this on my dual monitor setup, the JFrame spans only one of my monitors. I can make it work by replacing:
        frame.pack();
        frame.setVisible(true);
with:
       Dimension orig = frame.getMinimumSize();
frame.setMinimumSize(new Dimension(3000, 100));
        frame.pack();
        frame.setVisible(true);

This is a dirty trick that I don't expect to have to play. I posted this on StackExchange and a programmer there pointed out that it's the call to setVisible that is resetting the preferred size to fit the screen width.

Strangely enough, with this little demo, if I add frame.setMinimumSize(orig); after frame setVisible(true); the frame again fails to span my two screens.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Copy paste the first code block into a file, compile and run. It fails all the time on my machine. It works with the workaround I've suggested.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expect my frame to be wider than a single monitor.
ACTUAL -
The frame spans only one monitor in spite of the width of one of it's components exceeding the monitor width.

---------- BEGIN SOURCE ----------
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/* FrameDemo.java requires no other files. */
public class FrameDemo {
    /**
     * Create the GUI and show it. For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("FrameDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JLabel emptyLabel = new JLabel("blahblah");
        emptyLabel.setPreferredSize(new Dimension(3000, 100));
        frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
I can make this program work by replacing:
        frame.pack();
        frame.setVisible(true);
with:
       Dimension orig = frame.getMinimumSize();
frame.setMinimumSize(new Dimension(3000, 100));
        frame.pack();
        frame.setVisible(true);

This is a dirty trick that I don't expect to have to play. I posted this on StackExchange and a programmer there pointed out that it's the call to setVisible that is resetting the preferred size to fit the screen width.

Strangely enough, with this little example, if I add frame.setMinimumSize(orig); after frame setVisible(true); the frame again fails to span my two screens.


FREQUENCY : always