I should be able to only show the stacks.
The filter Hide stacked files shows all stacks as if they are collapsed.
Inverting it shows the files expanded but without the top of the stack.
What is needed is a filter to remove all files not part of a stack.
( I am sure I can set up a special category and filter on it - and I could also write a script to do it.
This is a request for a filter-panel entry.)