Run the attached program, make the window big enough to see, click at the
checkboxes. Short, quick clicks seem to cause the most problems.

Sometimes a mouse click doesn't select the box. This can result in a
situation where the box fills in with a dark spot as the pointer is moved
over top; and disappears when the pointer is moved away.

Mouse clicks on selected items can cause similar bizarre effects.

I have not checked the affect this strange visual behaviour has to on the
Java API for Checkbox and CheckboxGroup.