public static void paintParentBackground( Graphics g, JComponent c ) {
		Color background = getParentBackground( c );
		if( background != null ) {
			g.setColor( background );
			g.fillRect( 0, 0, c.getWidth(), c.getHeight() );
		}
	}
Fill background with parent's background color because the visible component
is smaller than its bounds (for the focus decoration).
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatUIUtils.java