ADDITIONAL SYSTEM INFORMATION :
Windows 11

A DESCRIPTION OF THE PROBLEM :
For some (most) non-integer UI scales, JScrollBar sliders leave behind "lines" when moved in the positive direction. This does not happen for scrollbars in a JScrollPane, and appears not to happen with the Windows Look&Feels. I have not tested on MacOS or Linux.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the code and run on Windows with: java -Dsun.java2d.uiScale=2.3 ScrollBarBug
Click and drag the scrollbar to the right.
Click the right arrow scrollbar button.

Note that if the screen scaling on Windows is already non-integer, this can happen when running without the uiScale specification.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expect normal scrollbar behavior.
ACTUAL -
Lines (copies of the edge of the scrollbar slider, I suppose) are left behind when moving the scrollbar in the positive direction. With a click-and-drag, they are cleaned up on release. With right arrow clicks, they remain. The slider also seems to change to a slightly incorrect size when dragging, then is restored to the correct size on release.

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

public class ScrollBarBug {
   public static void main(String[] args) {
      SwingUtilities.invokeLater(()->test());
   }
   public static void test() {
      JFrame f = new JFrame();
      JPanel panel = new JPanel(new BorderLayout());
      JScrollBar sb = new JScrollBar(JScrollBar.HORIZONTAL);
      panel.add(sb, "South");
      f.setContentPane(panel);
      f.setSize(800, 200);
      f.setVisible(true);
   }
}

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

FREQUENCY : always