Mostly, there's just relatively little interest from paying customers right now. PHI64, PHI675, PHI723 and PHI985 are all somewhat interested, but they're generally low-priority / "nice-to-have" sorts of requests, or this is a tangential or supplemental approach that might help with some other problem the request focuses on. Since there's limited customer interest, this isn't a very high priority right now.

Is this functionality blocked on something else?

This is many years old, hasn't seen further reports, and no longer has usable reproduction instructions.

Piece by piece, this was effectively untangled. Differential largely moved forward; Arcanist has a path forward in T13098 (although it's ultimately a "major version" sort of change), and Diffusion stuff has settled into relatively actionable plans in, e.g., T13075.

In my case (using GMail) it appears to thread properly as well however apparently some clients (KMail) rely on these headers more heavily than others.

In order for threading to work in email clients, the Message-ID header needs to be set on the initial email, with following emails having In-Reply-To / References set to that same Message-ID.

I can't reproduce this: mail threads properly in my client.

• D19634 allows reviewers to mark their own inlines as "Done" before they submit them. This addresses the case where you want to leave a non-actionable inline (like "This is much-improved.") but feel like leaving an inline like this is imposing on the author because they "must" check it off.
• D19635 marks the author's own inlines as "Done" by default. They may un-mark them if they prefer (e.g., actually want to leave themselves a TODO). This addresses the case where authors feel like they "must" check off every comment, so they either reply in two passes (one to submit; one to check off all their own replies) or avoid replying.

@bgamari Did you ever find a solution?

This is also showing up in initial email, which is silly and pointless.

I believe this was fixed by D17558 and discussed in T11050.

I've marked D19541 as resolving this. The change there isn't quite what was asked for here, but overlaps significantly.