No ScrolledEvent

I use SmartGWT 4.0 power version. For the following code, I am not able to get ScrolledEvent when the top layout is 100%.
@Override
public void onModuleLoad() {
VLayout layout = new VLayout();
layout.setWidth100();
layout.setHeight100();

// create something large enough to allow scrolling
Canvas canvas = new Canvas();
canvas.setHeight(3000);
layout.addMember(canvas);
layout.addScrolledHandler(new ScrolledHandler() {
@Override
public void onScrolled(ScrolledEvent event) {
SC.logWarn("scrolled");
}
});

layout.draw();
}