protected void frameStateChanged() {
		if( window == null || rootPane.getWindowDecorationStyle() != JRootPane.FRAME )
			return;

		updateVisibility();

		if( window instanceof Frame ) {
			Frame frame = (Frame) window;

			if( isWindowMaximized() &&
				!(SystemInfo.isLinux && FlatNativeLinuxLibrary.isWMUtilsSupported( window )) &&
				rootPane.getClientProperty( "_flatlaf.maximizedBoundsUpToDate" ) == null )
			{
				rootPane.putClientProperty( "_flatlaf.maximizedBoundsUpToDate", null );

				// In case that frame was maximized from custom code (e.g. when restoring
				// window state on application startup), then maximized bounds is not set
				// and the window would overlap Windows task bar.
				// To avoid this, update maximized bounds here and if it has changed
				// re-maximize windows so that maximized bounds are used.
				Rectangle oldMaximizedBounds = frame.getMaximizedBounds();
				updateMaximizedBounds();
				Rectangle newMaximizedBounds = frame.getMaximizedBounds();
				if( newMaximizedBounds != null && !newMaximizedBounds.equals( oldMaximizedBounds ) ) {
					int oldExtendedState = frame.getExtendedState();
					frame.setExtendedState( oldExtendedState & ~Frame.MAXIMIZED_BOTH );
					frame.setExtendedState( oldExtendedState );
				}
			}
		}
	}
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