I've noticed that masking/unmasking a panel that has a scrollbar would scroll that panel at the top of it.
From what i understand the TreePanel has a buffered view with the tree nodes which are to be displayed. When you scroll, using the mouse, the view is updated with the needed tree nodes.
Lets take the case when you have a TreePanel which has many tree nodes, and the panel is scrolled at its bottom, if you mask/unmask the component it will jump at its top. At this point you will get a white panel because the view of the tree doesn't get updated and no tree nodes will be rendered. If you move the scollbar manualy, the tree nodes are rendered back again.
I think a listener is needed in order to update the TreePanel rendering when the components scrolls automatically.