GlobalResourceHandler not cleaned up when UI gets detached

When an UI is closed, detached or times out. The GlobalResourceHandler is not cleaned up as cleanConnectorMap is never called again.

Demo

public class DemoUI extends UI {

	Label				label	= new Label("test");
	private final Field	usedResourcesField;

	public DemoUI() throws Exception {
	usedResourcesField = GlobalResourceHandler.class.getDeclaredField("usedResources");
		usedResourcesField.setAccessible(true);
	}

	@Override
	protected void init(final VaadinRequest request) {
		final Resource resource = new FileResource(new File("test"));
		getSession().getGlobalResourceHandler(true).register(resource, this);

		setPollInterval(1000);
		addPollListener(poll -> {

			final GlobalResourceHandler handler = getSession().getGlobalResourceHandler(false);
			Map<ClientConnector, ?> usedResources;
			final Collection<UI> activeUIs = getSession().getUIs();
			try {
				usedResources = (Map<ClientConnector, ?>) usedResourcesField.get(handler);

				label.setValue(
					Instant.now() + ". this is " + toString() + "; active resources: " + usedResources.keySet() + "; active uis: "
							+ activeUIs
				);
			} catch (final Exception e) {
				e.printStackTrace();
			}

		});
		setContent(label);
	}
}

Actual Behaviour

After opening two tabs and closing one. After some time the closed UI disappears in active uis but not in active resources.

Expected Behaviour

Also the resources are cleaned up.

Workaround

	@Override
	public void detach() {
		super.detach();
		getConnectorTracker().cleanConnectorMap(true);
	}

Runtime Environment

Vaadin Framework version: 8.9.3 (with compatibility classes)
Web container: Jetty 9.4.29

Start with GlobalResourceHandler and the UI detachment lifecycle, then inspect where ConnectorTracker.cleanConnectorMap(true) is called. Reproduce the two-tab DemoUI scenario from the issue and compare active UIs with active resources after closing one tab. Done means resources associated with the detached or timed-out UI are removed without requiring the workaround in UI.detach().