How to set vertical scroll bar in a panel

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1

How to set vertical scroll bar in a panel

        Panel panel = new Panel();
        panel.setWidth("225px");
        panel.setHeight("100%");
        panel.getContent().setSizeUndefined();
        panel.setScrollable(true);
        panel.addComponent(comboBox);
        panel.addComponent(tree);

If i set

 panel.setHeight("200px");

but i wants varying height (ie, when browser maximized one height and minimized one height)

how it possible

please give me solution