> You can do this in Fastmail (presuming you don't have the reading pane turned on, i.e. are in the same layout as Gmail).
EDIT: this message alerted me to the fact that there is a "hide reading pane" option so I am experimenting with that to see if this solves my problem. Also it would be SUPER helpful if the arrow keys would navigate through the message list like gmail does, J/K continues to be less comfortable for me.
If I check one of the messages/threads in the messages pane, such that I'm selecting it for being marked as read or deletion or something like that, in the right hand "read" pane, the message/thread on the right side turns into a single bar that is unreadable. in gmail I can select as many messages as I want and move back and forth between the "messages" and "Read thread" panes, using the enter key to read, semicolon to expand thread, and "u" to return to the message list, and the selections remain.
I have not been able to identify any similar flow in Fastmail; it forces me to deal with only one thread at a time, and I cannot read a message/thread that is also selected; clicking on it deselects it. So I can't select a series of messages and also read them, it would take a rethink of Fastmail's UX architecture for it to be possible.
> Shift-click a message header to apply to all at once, or "Shift-E" kb shortcut to expand all.
OK, Gmail's semicolon / colon toggle is much easier to use and also includes collapsing the thread also can this be added?
> We take a slightly different approach, but using Cmd-Shift-S will toggle you instantly between your draft and the thread you are replying to, preserving your scroll position in both.
OK, that is definitely more awkward. in gmail I can just scroll up to read the other messages in the thread, which is important because I often have to go back and read messages I'm responding towards. In fastmail, this would mean the "edit draft" button would give me an inline composition window that scrolls with the thread. This could be added to Fastmail with a lot of work but the UX at least does not prohibit this from being graphically feasible.