Sometimes, I have differences between web mode and hosted mode. Well I feel it looks like a GWT bug, but GXT code will probably not help GWT team. What can I do ? Here is the code :

public void onModuleLoad(){
        Viewport viewport = new Viewport();
        viewport.setLayout(new BorderLayout());

        BorderLayoutData centerData = new BorderLayoutData(LayoutRegion.CENTER);
        centerData.margins = new Margins(0, 0, 5, 0);
        ContentPanel mainPanel = new ContentPanel();

        BorderLayoutData mainCenter = new BorderLayoutData(Style.LayoutRegion.CENTER);
        mainCenter.margins = new Margins(0, 5, 5, 5);
        viewport.add(mainPanel, mainCenter);
        ToolBar topPanel = new ToolBar();
        final Menu menu1 = new Menu();
        menu1.add(new Item("This is a long long item text"));
        menu1.add(new Item("Show all files"));
        TextToolItem tool1 = new TextToolItem("File");
        BorderLayoutData northData = new BorderLayoutData(Style.LayoutRegion.NORTH,15);
        viewport.add(topPanel, northData);
        ContentPanel west = new ContentPanel();
        west.setHeading("My application");
        BorderLayoutData westData = new BorderLayoutData(Style.LayoutRegion.WEST, 200, 150, 350);
        viewport.add(west, westData);
The differences :
- the toolbar height : good looking in hosted mode, but I think more than 15 pixel height. 15 pixel height in web mode. As a workaround, I can set 25 pixel height and it's ok for both.
- the menu width : ok in hosted mode, not ok in web mode (text overflow)
- the margins

I have also noticed some differences in these threads :