This question already has an answer here:
It's annoying that on stack overflow ` ` works fine in questions and answers but not in comments, to produce a single code-formatted space.
Edit a few days later
Actually my question appears to be a duplicate of Markdown in comments: Inline code section does not format correctly if it contains only white-space which is marked as a duplicate of My comment `code block` was not converted properly, is this a bug? which in turn cites a comment by Jeff which is apparently no longer visible in the thread at Recent change of inline code in comments leads to mistakes in old comments
The duplicate-spotting code didn't locate these initially but some of the people who edited my question and/or posted relevant answers managed to include enough material to make the duplicate-spotter suggest it in the right sidebar.
So now it's all as clear as mud to me.