Wellllll... I have done several combinations of un- and re-install, and I think I have cornered the problem.
V 7-2 Preview Window Zoom apparently only works on the original computer that Sigil is/was installed on. I wiped Sigil from everywhere, and stripped the registry entries, and re-installed on my portable drive running on an XP machine. Everything works great.
When I move the portable drive to another computer, Preview Window Zoom no longer works. So I assume it must require some registry entry or specially installed component to operate correctly.
This is the first version that has not had full functionality when I move from computer to computer (it works portably, but is not "stealth" as the portable-apps folk call it, it does create settings and registry entries on each host computer you use), so no wonder I was baffled!
Since I am blocked from installing software on this computer, I will have to stay with v 7-1 on this one.
I wonder if the developers could pinpoint what component or registry entry is the culprit, maybe there is a way to re-direct that to Sigil's own folder so us "portable" fans can continue to use it on multiple computers.
EDITED TO ADD: Never mind.... I see that PortableApps.com does have a fully portable version available, I had not checked their site in forever.
So this has been a good mind-stretching exercise, and this entire post can be ignored. Sorry about that.