@Override
    public void insert(final Widget child, final Direction direction, final double size, final Widget before) {
        super.insert(child, direction, size, before);
        if (direction != Direction.CENTER) {
            insertSplitter(child, before);
        }
    }
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