@Override
		public void paintBorder( Component c, Graphics g, int x, int y, int width, int height ) {
			if( isWindowMaximized( c ) || FlatUIUtils.isFullScreen( c ) )
				return;

			Container parent = c.getParent();
			boolean active = parent instanceof Window && ((Window)parent).isActive();

			g.setColor( FlatUIUtils.deriveColor( active ? activeBorderColor : inactiveBorderColor, baseBorderColor ) );
			HiDPIUtils.paintAtScale1x( (Graphics2D) g, x, y, width, height, this::paintImpl );
		}
Window border used for non-native window decorations.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java