@Override
		public Insets getBorderInsets( Component c, Insets insets ) {
			if( isWindowMaximized( c ) || FlatUIUtils.isFullScreen( c ) ) {
				// hide border if window is maximized or full screen
				insets.top = insets.left = insets.bottom = insets.right = 0;
				return insets;
			} else
				return super.getBorderInsets( c, insets );
		}
Window border used for non-native window decorations.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java