Scrollbar not appearing when tree extends beyond bottom of screen

Here is the layout:

VerticalLayout with View
    VerticalLayout
        TabSheet with 4 tabs
            problem tab is a VerticalLayout
                Horizontal Layout
                    Button
                Panel
                    VerticalLayout
                        Tree
        
    Button(Logout)

When the tree extends beyond the bottom I can see tree items though the logout button at the bottom  - but no scroll bar.

I'm not sure where I should be putting the various sizeFull or sizeUndefined.

Vaadin 7.6.4