@Override
		public Insets getBorderInsets( Component c, Insets insets ) {
			insets.set( showBorder( c ) ? 1 : 0, 0, 0, 0 );
			return insets;
		}
Window border used for non-native window decorations.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java