>>> But it would be better to have there a test which would tell us 
>>> whether the region face is "visually different" from the default face. 
>>> Can we do something like that?
>>
>> face-differs-from-default-p?
>
> Nice catch; IIUC display-supports-face-attributes-p is what is doing the 
> heavy work.
>
> Might need to be made smarter wrt :extend though?
>

Indeed...

>
> (set-face-background 'region (face-background 'default nil t))
> (face-differs-from-default-p 'region nil)
> ; ⇒ :extend
>

... although that's a really contrived example.

I'm not sure it's TRT to add too much complexity there, so I would suggest 
to add an additional optional argument to face-differs-from-default-p, to 
exclude one or more attributes from the comparison (in this case :extend, 
but someone else might be interested in excluding other attributes in the 
future).