Portfolio frozen: can't view component documents

New problem that I've never seen before:  Acrobat won't allow scrolling to see the full list of documents in a portfolio.

I have a portfolio comprised of 50 documents.  For reasons I can't fathom, I am no longer able to scroll down the left side of the viewing window to view the full list of documents.  The list visibility is constrained to the size of the window:  elongating the screen (where possible) shows more documents in the list, shortening the window shows fewer.  It's not possible to scroll using a mouse or the arrow keys to view any documents beyond what is visible.  

Any idea why this problem just materialized out of thin air?  I've done no modification to the portfolio since my last viewing, when this standard feature worked just fine.  Any idea how to fix this?  I'm about to launch a product that requires that customers be able to view the whole list of component documents in the portfolio, so I need this problem like I need a hole in the head.  😞