@Override
		public void paintBorder( Component c, Graphics g, int x, int y, int width, int height ) {
			if( showBorder( c ) ) {
				float lineHeight = UIScale.scale( (float) 1 );
				FlatUIUtils.paintFilledRectangle( g, borderColor, x, y, width, lineHeight );
			}
		}
Window border used for non-native window decorations.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java