Hi Basj,
I tried the .bat script in the zip file you linked to, but my anti-virus software wouldn't entertain it. Saw it as a threat, "disinfected", and "quarantined" it.
So I tried a manual re-install, from a freshly downloaded installer. That didn't fix the anomaly either.
I've theorised that Fences probably heavily interacts programmatically with Windows File Explorer, and that this non-rolling-up anomaly may arise because I am running a File Explorer alternative -- the excellent Directory Opus ( https://www.gpsoft.com.au ) -- which intercepts most programmatic calls to File Explorer.
So I'll live with the "problem" of not having the roll-up facility unless the Fence sits at the edge of the desktop screen. Life's too short to spend hours trying to fix a minor problem that may possibly in any case result from using the File Explorer alternative.
Thank you for your help so far and I'll leave it here as an "open verdict" but otherwise closed case.