private void paintImpl( Graphics2D g, int x, int y, int width, int height, double scaleFactor ) {
			Object[] oldRenderingHints = FlatUIUtils.setRenderingHints( g );
			float lineWidth = (float) (UIScale.scale( 1f ) * scaleFactor);
			g.fill( FlatUIUtils.createRectangle( x, y, width, height, lineWidth ) );
			FlatUIUtils.resetRenderingHints( g, oldRenderingHints );
		}
Window border used for non-native window decorations.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java