Brian Vogel <britechguy@...>
How threading is presented is generally controllable within the e-mail client itself. Even in your example, the end result in following classic threaded view versus conversation view is whether the date sort is ascending or descending for all practical intents and purposes. Outlook can be set to use the "classic indented view" in conversations and give you the threaded view you show.
As has already been pointed out, threading is generally handled in different ways by different clients. But whatever the method, within a client the behavior is consistent. Threading can easily be broken in many where both subject and header information must match or where only subject must match. One tiny tweak to the subject line breaks threading, and that happens in Groups.io all the time. One of my primary tasks as a moderator is "re-threading" topics that have been accidentally split by one of the end users tweaking the subject line. It drives me utterly insane when there's been a topic on, say, "Knitting a Classic Cable Sweater," where at various times users do things like changing the subject, mid-topic, to something like, "I know this tip for making this sweater!," or, "Thanks for all the help" at the end. Long term users of e-mail lists and online forums both were taught, at one time, that you never, ever, ever do this.
As a general rule, I believe in educating users to the conventions that have been commonly in use for decades now for communities such as this one. Others wish to come up with ways for the software to "figure out" how to manage behaviors [as far as subject changes, etc.] that they shouldn't have to manage in the first place. In my experience, all that does is make things messier, and messier, and messier as people keep doing all sorts of stuff they need to know that you just don't do.
Cyber society, like society at large, has conventions that those entering it are obligated to learn and use. Their own choices of platforms imposes the obligation on them to learn how to accomplish the same thing multiple ways.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
I can hire one half of the working class to kill the other half.
~ , U.S. financier & railroad robber baron (1836 - 1892)