For what it's worth, Mailbird and Sparrow do this.  It's a little disorienting not to have these if you're using labels as true labels, rather than pseudo-folders.  A conversation can have multiple labels/tags, but there's no way to see this in the app right now.  Just wanted to lend my voice to the cause.

I'm interested in taking up this bug as I use normally use this very often, but I would like to know if other people wouldn't mind. I wouldn't implement it in the conversation list yet, but in the conversation viewer it would be a nice addition (somewhere on top maybe, like in Gmail?)

This would indeed be good to have. It needs some design work though, so can we workshop some use cases for it? Might be worth asking aday and/or #gnome-design Here's a few to start:

Alice uses labels as tags/topics. When a new message arrives in her Inbox, she wants to know which labels a specific conversation have already been applied, to first remind her and later to allow correctly labelling of new messages in the same conversation.

Bob uses labels for specific people. He wants to quickly find messages from them or related to them.

Chau is searching for a conversation, and in the results comes across a conversation that should be labelled in a specific way. She wants to know if the conversation is labelled correctly or not.

Denali uses his Inbox as a TODO list. He wants to know what messages in a conversation have been archived, and which still need to be actioned.

Ehsan simply uses Inbox, Archive, Drafts, Sent. When a conversation is read and maybe replied to, it gets archived.

Any others?

I feel like Alice and Chau probably want to see a summary of labels for the whole conversation, while Denali wants to see labels for individual messages.

Does Bob even care about seeing the labels? Maybe "Search for messages from..." and/or Bug 771849 would care of his labelling approach automatically, or at least in part?

Both Alice and Bob might want to switch to the mailbox for the specific label to see other related messages, so maybe the labels should actually be buttons?

Alice may benefit from having an Archive button that like Ehsan, she can just click to have new messages labelled correctly, but applies labels from the other messages in the convo.

See also notes up on the wiki at: https://wiki.gnome.org/Apps/Geary/Design/ConversationLifecycle

Created attachment 368365 [details]
Mockup showing labels in conversations

An attempt at a simple enhancement of the existing UI to also display the folders/labels that a conversation's messages exist in.

Source SVG is in the wip/ux branch: https://git.gnome.org/browse/geary/tree/ux/conversations?h=wip/ux

Hey Alan, could I get your opinion on the attached mockups?

I was originally thinking something snazzier like a tag icon or surrounding the text with a thin border to make it look like a physical tag or something, but just using the position of the text as in the mockups seemed to work well by itself. They could grow coloured swatches or backgrounds in the future if Geary started supporting those.

Of the four options, I think (3) is probably the way to go, the ellipsising of the recipient addresses won't be as bad if the GtkFlowBox's minimum column count is relaxed back to 1 (it's currently set to 2 to ensure the list isn't /too/ long when there are many recipients).

Still need to look into how these would fit into the conversation list, but I suspect it would work in a similar sort of way as for the collapsed messages above.

Thinking about this a bit more, we probably don't want to show labels for every conversation in the conversation list, since each list item already has a lot of text innformation and we would likely need to use an extra line for it, reducing the number that can be shown at once. We could instead show it in the GtkHeaderBar for the selected conversations if we start showing the subject line there, which is the case for Bug 730682. I.e. Show the subject as the header bar title, and the set of labels for the conversation as the subtitle.

An issue with the mockups above is that they all assume the Subject will continue to be shown for every message, however that isn't necessarily the case (Bug 714025) and with the "Additional Detail" design (Bug 713727) it will indeed likely be not displayed by default.

The results for the user survey about piling vs filing are starting to show that not many people file in more than one folder, so having to display multiple labels is perhaps less of a priority. The labels could still be quite long however since per the survey a substantial number of people file in per-sender.

Despite Subject going away by default, mockup #3 might still work with enough screen space and judicious use of ellipses. This means that a message with a single recipient would only take two lines for the header, even when not collapsed.

With the Subject going away, Mockup #4 would take extra vertical space when there is only one or two recipients, making it it look much less like a viable option. Perhaps it could be used when there are more than one or two recipients, it would be nice if GTK+ could take care of this for us though.

GNOME is going to shut down bugzilla.gnome.org in favor of gitlab.gnome.org.
As part of that, we are mass-closing older open tickets in bugzilla.gnome.org
which have not seen updates for a longer time (resources are unfortunately
quite limited so not every ticket can get handled).

If you can still reproduce the situation described in this ticket in a recent
and supported software version, then please follow
  https://wiki.gnome.org/GettingInTouch/BugReportingGuidelines
and create a new ticket at
  https://gitlab.gnome.org/GNOME/geary/-/issues/

Thank you for your understanding and your help.