@Override
	public void startScroll(ScrollEvent event) {
		this.viewportPolicy = determineViewportPolicy();
		init(viewportPolicy);
		// delegate to scroll() to perform panning/zooming
		scroll(event);
	}

	public void scroll(ScrollEvent event) {
		// each event is tested for suitability so that you can switch between
		// multiple scroll actions instantly when pressing/releasing modifiers
		if (isPan(event)) {
			pan(event);
		} else if (isZoom(event)) {
			zoom(event);
		}
	}

	protected void pan(ScrollEvent event) {
		// Determine horizontal and vertical translation.
		Dimension delta = computePanTranslation(event);
		// change viewport via operation
		viewportPolicy.scroll(true, delta.width, delta.height);
		// restrict panning to contents
		if (isContentRestricted()) {
			panningSupport.removeFreeSpace(viewportPolicy, Pos.TOP_LEFT, true);
			panningSupport.removeFreeSpace(viewportPolicy, Pos.BOTTOM_RIGHT,
					false);
		}
	}

	public void endScroll() {
		commit(viewportPolicy);
		this.viewportPolicy = null;
	}

if (host instanceof IRootPart) {
			// check if click on background (either one of the root visuals, or
			// an unregistered visual)
			if (!isRegistered(e.getTarget())
					|| isRegisteredForHost(e.getTarget())) {
				// click on background