GWT:browser scrolling not appearing - gwt

Question

I am having a small application in GWT , the main page is having the initWidget as VerticalPanel and inside there are just other VerticalPanel and horizontalPanels.
But I am not able to see browser scrolling. The verticalpanels are going beyond browser height but there is scrollbars appearing .
Any idea what could be the reason please.
thanks
code::
private Button btnLoad = new Button("load more results");
private CheckBox check_byPakag= new CheckBox("by package");
private CheckBox check_Inheritance= new CheckBox("by inheritance");
private CheckBox check_byComposition= new CheckBox("by composition");
private CheckBox check_byDependency= new CheckBox("by dependency");
private VerticalPanel vpnl_Main = new VerticalPanel();
private FlowPanel flowpanelImages = new FlowPanel();
private VerticalPanel scrollImages;
private IndexServiceAsync rpcService = GWT.create(IndexService.class);
private SuggestBox text_Wild_Card;
private MultiWordSuggestOracle oracle = new MultiWordSuggestOracle();
private int start = 0;
private int finish = 10; // Total number of results to display at first time
private int totalNumberOfResultsToShow = 10; //Total number of results to display on every call(when load more results button is pressed)
private int scrollPosition = 0;
private VerticalPanel vpnlScroll = new VerticalPanel();
public MainPageView(){
check_byPakag.setChecked(true);
check_Inheritance.setChecked(true);
check_byComposition.setChecked(true);
check_byDependency.setChecked(true);
initWidget(vpnl_Main);
getSuggestions();
scrollImages = new VerticalPanel();
scrollImages.add(vpnlScroll);
vpnlScroll.add(flowpanelImages);
btnLoad.setEnabled(false);
vpnl_Main.setSpacing(4);
Window.addResizeHandler(new ResizeHandler(){
#Override
public void onResize(ResizeEvent event) {
}});
}
public void getSuggestions(){
rpcService.getAllSuggestions(new AsyncCallback<List<String>>(){
#Override
public void onFailure(Throwable caught) {
System.out.println(caught.getMessage());
}
#Override
public void onSuccess(List<String> result) {
for(int i=0;i<result.size(); i++){
oracle.add(result.get(i));
}
text_Wild_Card = new SuggestBox(createWildCardOracle());
text_Wild_Card.setText("*");
layout(vpnl_Main);
getIndexData();
}});
}
#SuppressWarnings("deprecation")
public void getIndexData(){
final DecoratedPopupPanel popup = new DecoratedPopupPanel();
popup.setWidget(new Label("Loading.."));
popup.center();
rpcService.getIndexData(text_Wild_Card.getText(), check_byPakag.isChecked(), check_Inheritance.isChecked(), check_byComposition.isChecked(), check_byDependency.isChecked(), start, finish, new AsyncCallback<IndexDataSet>(){
#Override
public void onFailure(Throwable caught) {
Window.alert("getIndexData failed"+ caught.getMessage());
}
#Override
public void onSuccess(IndexDataSet result) {
flowpanelImages.clear();
if(result.hasMore()){
btnLoad.setEnabled(true);
}else{
btnLoad.setEnabled(false);
}
for(int i=0; i< result.getResults().size(); i++){
final Image image = new Image(result.getResults().get(i).stampImageURL);
//////////////
final Label lbl = new Label("dummy");
lbl.setStyleName("invisibleImageLabel");
VerticalPanel vpnlImage = new VerticalPanel();
vpnlImage.add(image);
vpnlImage.add(lbl);
//////////
final DataImage dataImage = new DataImage();
dataImage.setUrl(result.getResults().get(i).stampImageURL);
dataImage.setDescription(result.getResults().get(i).description);
image.addMouseOverHandler(new MouseOverHandler(){
#Override
public void onMouseOver(MouseOverEvent event) {
lbl.setText(dataImage.getDescription());
lbl.setStyleName("imageLabel");
}});
image.addMouseOutHandler(new MouseOutHandler(){
#Override
public void onMouseOut(MouseOutEvent event) {
lbl.setStyleName("invisibleImageLabel");
}});
image.addClickHandler(new ClickHandler(){
#Override
public void onClick(ClickEvent event) {
RootPanel.get("bodyContainer").clear();
History.newItem("diagramView / "+dataImage.getUrl());
}});
vpnlImage.addStyleName("imageStyle");
vpnlImage.addStyleName("paddedFlowPanel");
flowpanelImages.add(vpnlImage);
}
//
HorizontalPanel hpnlBtn = new HorizontalPanel();
hpnlBtn.setWidth("100%");
hpnlBtn.setHorizontalAlignment(HasHorizontalAlignment.ALIGN_CENTER);
hpnlBtn.add(btnLoad);
btnLoad.setStyleName("nextLine");
flowpanelImages.add(hpnlBtn);
popup.removeFromParent();
}});
}
private SuggestOracle createWildCardOracle() {
return oracle;
}
private void layout(VerticalPanel vpnl_Main) {
FlexTable flexWildCard = new FlexTable();
VerticalPanel vpnlCheckBoxes = new VerticalPanel();
HorizontalPanel hpnlWildCard = new HorizontalPanel();
flexWildCard.setWidget(0,0,new Label("Show only class diagrams containing classes named: (use * for wildcard)"));
flexWildCard.setWidget(0,1,text_Wild_Card);
text_Wild_Card.setWidth("200px");
vpnlCheckBoxes.setWidth("200px");
vpnlCheckBoxes.add(check_byPakag);
vpnlCheckBoxes.add(check_Inheritance);
vpnlCheckBoxes.add(check_byComposition);
vpnlCheckBoxes.add(check_byDependency);
hpnlWildCard.add(vpnlCheckBoxes);
hpnlWildCard.add(flexWildCard);
vpnl_Main.add(hpnlWildCard);
}