FWIW the answer to this is pretty mundane.
I had a default Border style set in the app and the ControlTemplate for the TreeViewItem wraps the ToggleButton in two Borders....

>In my TreeView the 'Expander' ToggleButton is getting chopped to about half of the height required to display it completely - and I don't understand why.
>
>If I drill in using Snoop I can see that it's Height is in fact hard coded at 13 but nothing (AFAICS) is constraining the height of the containing TreeViewItem - so why is it getting chopped?
>
>TIA,
>Viv