Thanks.
Looking in Cal6 portable, I do see a blank progbar briefly, but only until the first story finishes metadata, then it's normal.
FFF already has some special code (that I had forgotten about) to basically 'skip' the first progbar loop with a comment about Win10 not showing the dialog content until it's looped once.
On a whim, I changed it to skip 2 loops first and that seems to work in Cal6 on win10.
It is, however, extremely kludgy. I want to research more before declaring that a solution.
Plus, I'm not able to upload zips to Mobile Read right now. No idea what that's about...