This is controlled via the Layout Setup dialog, ticking the option to "Delineate fields on the current record only" (which is applied in list view).
If your theme doesn't have a visible field border around inactive fields, then you may need to edit the theme to add the boundary lines you want.
Thank you very much for that, could have gone loopy trying to find that! (or should I say loopier??)
Yep, as FM gains in tools, more and more of them seem to be hidden where they are harder to find.
There are now so many controls that can only be reached via the Inspector tabs (now 4 of those) that it's a study in itself. I look back to when the Inspector was new, and I'm just now realizing that it was a fix for an interface where the pull-down menus had become overly layered and complex, and the panels provided room to keep expanding the tools.
I'm now using a wide screen monitor but am running out room to keep 4 inspector windows open while in layout mode with a layout designed for computer use!