A fully threaded view is one of the biggest missing features in an otherwise great email client.

Thanks for the feature suggestion, but Geary is designed around conversations, so a threaded view does not make sense in that context. There's no point in trying to duplicate something that other mail clients already do better. Thunderbird might be a good option, for example, for people looking for a threaded view.

Conversations in groups on the internet develop in parallel, with different parts of the discussion expanded in overlapping ways. Understanding exactly what part of the conversation was replied to in this context is essential.

Thunderbird is bloated and buggy, and doesn't seem to be actively developed anymore. Geary is so very close to being a perfect replacement, but it's many improvements are sadly eclipsed by the lack of sensible options for a threaded view.

Well, that's part of the problem - there are no sensible ways of displaying a conversation as a threaded view except by using a hierarchical, tree-based widget like GtkTreeView. A tree widget is not appropriate however since it doesn't fit in at all well with a conversation-based model, it doesn't scale for deeply nested conversations or on narrow displays, and it's also exceptionally hard for people to use - this is the reason why applications like mail clients and file browsers have all moved away from using trees for e.g. displaying threads or nested folders in recent years.

I'd be open to work-shopping alternative ways of displaying threads within the existing conversation model - maybe having an option to only display responses to a specific message in the conversation viewer for example, filtering or grouping by immediate replies, and navigating between sub-threads. If you'd like to send a message to the mailing list with your uses cases for a threaded view, I'd be happy to discuss it further there.