@Override
    public void setWidgetHidden(Widget widget, boolean hidden) {
        super.setWidgetHidden(widget, hidden);
        // If we are hiding a widget then we also need to hide its splitter else you get an
        // orphaned splitter appearing
        if (!(widget instanceof Splitter)) {
            final Splitter associatedSplitter = getAssociatedSplitter(widget);
            if (associatedSplitter != null) {
                setWidgetHidden(associatedSplitter, hidden);
            }
        }
    }
The element that masks the screen so we can catch mouse events over
iframes.
stroom-core-client-widget/src/main/java/com/google/gwt/user/client/ui/ThinSplitLayoutPanel.java