To reproduce, run the following on macOS 10.15.4 Catalina:

1. Run SwingSet2 as follows:

$ java -Dsun.java2d.metal=true -jar SwingSet2.jar

2. Click on the "Source code" button to show the source code of the demo in a JEditorPane.

3. Using a two-finger scroll gesture on the trackpad, scroll the source code pane.

BUG: a rectangular region in the middle of the window is not repainted while you are scrolling nor after you stop scrolling.

If you refresh the window in some way it will repaint correctly.

I can't reproduce it on my 10.13.6 High Sierra system.

This might be a 10.15 Catalina specific bug, or could be because I'm running on older hardware.