Now I got it.
The easy recipe: compare two files identical, were it not that one has tabs while the other has spaces instead.
Not ignoring white space, the two files of course are different.
Indeed, if you put the tabbed one in the left pane and the untabbed in the right pane (and selecting detail level: lines and chars) then you can see the "added" spaces.
Swap the files and... you can't see them anymore.
(N.B. if you swap again then the difference returns visible.)
(N.B. #2: It only depends on which pane contains which file; the swap is simply the easiest way to do the check.)
There is also something similar when saving a file in which you just copied a block of lines without tabs on the correspondent block in the other file, but the thing is more complicated and I've not yet studied it.
Food for later...
I tried resetting all pages to default, then, of course, I choose "lines and characters": the effect is still there.