What kind of comma is at play in this sentence from Middlemarch?

Question

I had what I esteem was a really basic education in English grammar, so I’m a bit puzzled about the comma before ”that Will Ladislaw” in this sentence from George Eliot’s Middlemarch:

Perhaps, through all other hidden thoughts, the one that breathed most comfort was, that Will Ladislaw at least was not likely to publish what had taken place that evening.

After some basic research, the explanation I’ve considered is that commas before restrictive clauses used to be more common in written English back when the book was published. I’ve also observed commas before “that” in Polish grammar when I dabbled in it, but in English texts it seems a bit odd.

Would anyone here have a more confident answer?

Answer 1

... that Will Ladislaw at least was not likely to publish what had taken place that evening.

The above is not a "restrictive clause" but a clause with nominal, noun-like, force, a content-clause, the predicate complement of "the one [thought] that breathed most comfort ^restrictive was, [that Will Ladislaw at least was not likely to publish what had taken place that evening].^{content clause}

The sentence could be rewritten

That Will Ladislaw at least was not likely to publish what had taken place that evening was, perhaps, the one thought through all other hidden thoughts that breathed most comfort.

A pause after was isolates the content-clause. I think the isolation reinforces the nominal role of the clause.

Nowadays we often hear copula was "attached" to the front of the complement:

The one thing that most puzzled me of all the events of that evening, was that they took so long to call the police.

A pause before "was" and "was that" spoken in rapid succession. At least that's the prosody in the American English I hear every day.