ADDITIONAL SYSTEM INFORMATION :
Dual monitor setup
Windows 10 
jre1.8.0_281

A DESCRIPTION OF THE PROBLEM :
A Simple undecorated JFrame does not fit into second monitor in some conditions which will be explained in detail below.
To fit frame in to the screen, I simply calculate "maximized bound" using monitors' bounds minus insets values.

setMaximizedBounds(getMaximizedBounds()); // calculate bounds
setExtendedState(MAXIMIZED_BOTH);
......
maxBounds = new Rectangle(screenInsets.left, screenInsets.top,
					bounds.width - screenInsets.right - screenInsets.left,
					bounds.height - screenInsets.bottom - screenInsets.top);



STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
=============================
TEST1:
Primary screen: 1920x1080 %100 scailing, rendered bound: 1920x1080
Second screen: 1280x1024 %100 scailing, rendered bound: 1280x1024
NO PROBLEM, JFrame fits in both screens. getBounds() returns 1920x1040 for the first monitor and 1280x984 for the second monitor
=============================
TEST2:
Primary screen: 1920x1080 %150 scailing, rendered bound: 1280x720
Second screen: 1280x1024 %100 scailing, rendered bound: 1280x1024
PROBLEM, JFrame does not fit into second monitor. getBounds() returns same bound value 1280x680 for both monitor .
=============================
TEST3:
Primary screen: 1280x1024 %100 scailing, rendered bound: 1280x1024
Second screen: 1920x1080 %100 scailing, rendered bound: 1920x1080
PROBLEM, JFrame does not fit into second monitor. getBounds() returns same bound value 1280x984 for both monitor .
=============================
TEST4:
Primary screen: 1280x1024 %100 scailing, rendered bound: 1280x1024
Second screen: 1920x1080 %150 scailing, rendered bound: 1280x720
NO PROBLEM, JFrame fits in both screens. getBounds() returns 1280x984 for the first monitor and 1280x680 for the second monitor
=============================
TEST5:
Primary screen: 1920x1080 %150 scailing, rendered bound: 1280x720
Second screen: 1920x1080 %100 scailing, rendered bound: 1920x1080
PROBLEM, JFrame does not fit into second monitor. getBounds() returns same bound value 1280x680 for both monitor .
=============================
TEST6:
Primary screen: 1920x1080 %125 scailing, rendered bound: 1920x1080
Second screen: 1920x1080 %100 scailing, rendered bound: 2400x1350
PROBLEM, JFrame does not fit into second monitor. getBounds() returns same bound value 1920x1030 for both monitor .


In all scenarios, the application has been restarted after monitor resolution change in order to avoid any auto-scailing issues.



ACTUAL -
Inconsistent monitor bounds.


---------- BEGIN SOURCE ----------
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.Insets;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

public class FrameTest extends JFrame {

	private JPanel contentPane;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					FrameTest frame = new FrameTest();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	public FrameTest() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 800, 600);
		contentPane = new JPanel();
		contentPane.setBackground(Color.YELLOW);
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		contentPane.setLayout(null);

		FrameDragListener frameDragListener = new FrameDragListener(this);
		addMouseListener(frameDragListener);
		addMouseMotionListener(frameDragListener);

		JButton btnNewButton = new JButton("Toggle");
		btnNewButton.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				if (getExtendedState() == MAXIMIZED_BOTH)
					setExtendedState(NORMAL);
				else {
					setMaximizedBounds(getMaximizedBounds());
					setExtendedState(MAXIMIZED_BOTH);
					System.out.println("rendered bounds => " + getBounds());
				}
			}
		});
		btnNewButton.setBounds(21, 21, 74, 34);
		contentPane.add(btnNewButton);

		JButton btnNewButton_1 = new JButton("X");
		btnNewButton_1.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				dispose();
			}
		});
		btnNewButton_1.setBounds(104, 21, 43, 34);
		contentPane.add(btnNewButton_1);
		setUndecorated(true);
	}

	public static class FrameDragListener extends MouseAdapter {

		private final JFrame frame;
		private Point mouseDownCompCoords = null;

		public FrameDragListener(JFrame frame) {
			this.frame = frame;
		}

		public void mouseReleased(MouseEvent e) {
			mouseDownCompCoords = null;
		}

		public void mousePressed(MouseEvent e) {
			mouseDownCompCoords = e.getPoint();
		}

		public void mouseDragged(MouseEvent e) {
			Point currCoords = e.getLocationOnScreen();
			frame.setLocation(currCoords.x - mouseDownCompCoords.x, currCoords.y - mouseDownCompCoords.y);
		}
	}

	public Rectangle getMaximizedBounds() {

		GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
		GraphicsDevice[] gs = ge.getScreenDevices();
		Rectangle bounds = this.getGraphicsConfiguration().getBounds();
		Rectangle maxBounds = null;
		Insets screenInsets = this.getToolkit().getScreenInsets(this.getGraphicsConfiguration());

		if (gs[0] == getGraphicsConfiguration().getDevice()) { // main monitor
			maxBounds = new Rectangle(screenInsets.left, screenInsets.top,
					bounds.width - screenInsets.right - screenInsets.left,
					bounds.height - screenInsets.bottom - screenInsets.top);
			System.out.println(getGraphicsConfiguration().getDevice().getIDstring() + " bounds " + bounds.width + "x"
					+ bounds.height + " ScreenInsets: " + screenInsets);
		} else if (gs[0] != getGraphicsConfiguration().getDevice()) { // other monitors
			maxBounds = new Rectangle(screenInsets.left, screenInsets.top,
					bounds.width - screenInsets.right - screenInsets.left,
					bounds.height - screenInsets.bottom - screenInsets.top);
			System.out.println(getGraphicsConfiguration().getDevice().getIDstring() + " bounds " + bounds.width + "x"
					+ bounds.height + " ScreenInsets: " + screenInsets);
		}

		else {
			maxBounds = null;
		}
		return maxBounds;
	}
}

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

CUSTOMER SUBMITTED WORKAROUND :
I overrided setExtendedState to solve scaling problems except scenario in TEST6. If you notice, scailing in primary monitor affects secondary monitor.

@Override
	public void setExtendedState(int state) {
		if (state == MAXIMIZED_BOTH) {
			// calculate setMaximizedBounds
			GraphicsConfiguration graphicsConfiguration = getGraphicsConfiguration();
			Rectangle screenBounds = graphicsConfiguration.getBounds();
			Insets screenInsets = this.getToolkit().getScreenInsets(this.getGraphicsConfiguration());

			Rectangle maxBounds = new Rectangle(screenInsets.left, screenInsets.top,
					screenBounds.width - screenInsets.left - screenInsets.right,
					screenBounds.height - screenInsets.top - screenInsets.bottom);

			setMaximizedBounds(maxBounds);
			super.setExtendedState(MAXIMIZED_BOTH);
			System.out.println("maxBounds => " + maxBounds);
			Rectangle bounds = getBounds();

			// since Java 9 we need calculate the scale factor because setMaximizedBounds
			// reduce the resulting bounds with the display scale factor
			if (bounds.width != maxBounds.width || bounds.height != maxBounds.height) {
				double factorX = (double) maxBounds.width / bounds.width;
				double factorY = (double) maxBounds.height / bounds.height;
				maxBounds = new Rectangle(screenInsets.left, screenInsets.top,
						(int) Math.round(screenBounds.width * factorX) - screenInsets.left - screenInsets.right,
						(int) Math.round(screenBounds.height * factorY) - screenInsets.top - screenInsets.bottom);
				setMaximizedBounds(maxBounds);

				super.setExtendedState(ICONIFIED); // state must change that the new MaximizedBounds is used from the
													// GUI
				SwingUtilities.invokeLater(() -> {
					super.setExtendedState(MAXIMIZED_BOTH);
					System.out.println("rendered bounds after scaling => " + getBounds());
				});
			}
		} else {
			super.setExtendedState(state);
		}
	}

FREQUENCY : always