SortingFocusTraversalPolicy uses ROW_TOLERANCE conception to alter a disposition of components in a focus cycle for the sake of visually more appropriate traversal order. This however breaks the transitivity rule, which in case of using the default tim-sort algo leads to an exception:

java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeHi(TimSort.java:895)
at java.util.TimSort.mergeAt(TimSort.java:512)
at java.util.TimSort.mergeForceCollapse(TimSort.java:453)
at java.util.TimSort.sort(TimSort.java:250)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1466)
at java.util.Collections.sort(Collections.java:175)
at javax.swing.SortingFocusTraversalPolicy.enumerateAndSortCycle(SortingFocusTraversalPolicy.java:172)
at javax.swing.SortingFocusTraversalPolicy.getFocusTraversalCycle(SortingFocusTraversalPolicy.java:143)
at javax.swing.SortingFocusTraversalPolicy.getFirstComponent(SortingFocusTraversalPolicy.java:500)
at javax.swing.LayoutFocusTraversalPolicy.getFirstComponent(LayoutFocusTraversalPolicy.java:167)
at javax.swing.SortingFocusTraversalPolicy.getDefaultComponent(SortingFocusTraversalPolicy.java:590)
at java.awt.FocusTraversalPolicy.getInitialComponent(FocusTraversalPolicy.java:169)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:420)
at java.awt.Component.dispatchEventImpl(Component.java:4760)
at java.awt.Container.dispatchEventImpl(Container.java:2302)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:751)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:702)
at java.awt.EventQueue$3.run(EventQueue.java:696)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:724)
at java.awt.EventQueue$4.run(EventQueue.java:722)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:190)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:115)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:104)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:100)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:92)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:81)

Run the following test case to reproduce it:

import java.awt.*;

import javax.swing.*;

public class TestLayoutBug extends JFrame
{
  private static TestLayoutBug dia;
  
  private static int[] Xs = new int[] {71, 23, 62, 4, 79, 39, 34, 9, 84, 58, 30, 34, 38, 15, 69, 10, 44, 95, 70, 54,
    44, 62, 77, 64, 70, 83, 31, 48, 96, 54, 40, 3, 60, 58, 3, 20, 94, 54, 26, 19, 48, 47, 12, 70, 86, 43, 71, 97, 19,
    69, 90, 22, 43, 76, 10, 60, 29, 49, 9, 9, 15, 73, 85, 80, 81, 35, 87, 43, 17, 57, 38, 44, 29, 86, 96, 15, 57, 26,
    27, 78, 26, 87, 43, 6, 4, 16, 57, 99, 32, 86, 96, 5, 50, 69, 12, 4, 36, 84, 71, 60, 22, 46, 11, 44, 87, 3, 23, 14,
    43, 25, 32, 44, 11, 18, 77, 2, 51, 87, 88, 53, 69, 37, 14, 10, 25, 73, 39, 33, 91, 51, 96, 9, 74, 66, 70, 42, 72,
    7, 82, 40, 91, 33, 83, 54, 33, 50, 83, 1, 81, 32, 66, 11, 75, 56, 53, 45, 1, 69, 46, 31, 79, 58, 12, 20, 92, 49,
    50, 90, 33, 8, 43, 93, 72, 78, 9, 56, 84, 60, 30, 39, 33, 88, 84, 56, 49, 47, 4, 90, 57, 6, 23, 96, 37, 88, 22, 79,
    35, 80, 45, 55};

  public TestLayoutBug()
  {
    JPanel panel = new JPanel(new GridBagLayout());
    GridBagConstraints gbc = new GridBagConstraints();
    for (int i=0; i < Xs.length; i++) {
      gbc.gridx = Xs[i];
      gbc.gridy = 100-gbc.gridx;
      panel.add(new MyComponent(), gbc);
    }
    getRootPane().getContentPane().add(panel);
    pack();
    setDefaultCloseOperation(EXIT_ON_CLOSE);
  }
  
  public static void main(String[] args) throws Exception
  {
    //System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");
    SwingUtilities.invokeLater(new Runnable() {

      @Override
      public void run()
      {
        dia = new TestLayoutBug();
        dia.setVisible(true);
      }
    });
  }

  public static class MyComponent extends JPanel
  {
    private final static Dimension SIZE = new Dimension(1,1);
    
    public MyComponent()
    {
      setBackground(Color.BLACK);
      setOpaque(true);
    }

    @Override
    public Dimension getPreferredSize()
    {
      return SIZE;
    }
  }
}