Users are not computers, They use their computer. Like everybody, i'm not able to remember specific shortcuts of hundreds of softwares.
It is one of the rules of good programming: you can only implement short cut as an add on. All the features had to be accessible by menus or symbols and mouse actions.
We are in a graphical interface; most of the features had to be available in a graphical and intuitive way. I asked this features to be on clicking the + symbol. This has no inconvenience, as opening only the next link to the nex message is just stupid and useless. This is obvious. You confirm the feature exists, so Why the devs does not satisfies after so many years this so simple request ?

This has to be that way: clicking to the "+" open all the links of this conversation in a threaded view (and the plus transform themself in a "-"). Clickink the "-" close what is under. Clicking the + of the next discussion close the previous thread if opened and open this next one.
Opening all the views at once is stupid too, because it takes a lot of room, and oblige-you to scroll miles.

Sorry, but i do not agree. All the programs using threaded views (specially Usenet readers like Xnews, gravity etc...) are  working the way i describe-it (i had edited my previous message) . Messages are not folders, they are single messages. And threathed view is for see how is organised a discussion in a single sight.
Opening all threads by default is just a pain, when you have hundreds of messages, as  it oblige-you to scroll during hours. And who wants to see how are organised already read discussions ?
You see a discussion with an unread message: you want to open-it, and see, in one sight how was the discussion. In this discussion there is an unread message ? you open-it in a click.
Then, you open the next unread discussion: it close the previous one with no more interest for you.
It is the well known way to manage threaded views, way before Internet exists. Why don't respect the habits when they have proven their perfect ergonomic ?