Hmm.
The question that was asked was what was legal. I don't know of any legal requirement to be able to glide clear of fog. Rules about maintaining a given distance from cloud aren't relevant if you're sufficiently far above the fog or sufficiently low that the rules don't apply.
The only law I can think of that's relelvant is that you must be "in sight of the surface". And, from a legal point of view, surely that's all that's required - to be in sight of the surface? Doesn't matter what bit of surface it is, if you can see it, you're legal.
As is so often the case, though, what's legal and what's safe aren't necessarily the same. There's the ability to glide safely(but not if you're VFR in a twin without an IMC rating or IR). And there's the ability to navigate safely. And probably some other safety issues which aren't actually covered by the law but which are common sense.
FFF
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