static String getWindowStyle( JRootPane rootPane ) {
		Window w = SwingUtilities.getWindowAncestor( rootPane );
		String defaultWindowStyle = (w != null && w.getType() == Window.Type.UTILITY) ? WINDOW_STYLE_SMALL : null;
		return clientProperty( rootPane, WINDOW_STYLE, defaultWindowStyle, String.class );
	}
This panel paint a border at the top of the window in fullWindowContent mode,
if FlatLaf window decorations are enabled.
Only used on Windows 10.
<p>
This panel is not a child of the title pane.
Instead it is added by FlatRootPaneUI to the layered pane at a layer over all other layers.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTitlePane.java