Skin adds the line but never removes it.

failing test:

    @Test
    public void testControlChildren() {
        installDefaultSkin(control);
        int childCount = control.getChildrenUnmodifiable().size();
        replaceSkin(control);
        assertEquals("skin must remove direct children that it has added", 
                childCount, control.getChildrenUnmodifiable().size());
    }
    
fix: remove in dispose