A reply that stays in its thread counts only there: the channel or direct message keeps its place in the sidebar, the app icon and Unreads, and a reply also sent to the channel counts as before. Threads is drawn bold, with a dot on the collapsed rail, as soon as a thread you follow has a new reply, and clears when you read it on any of your devices. A partner organization’s people see their threads in shared channels there too.
The draft of a thread’s first reply is kept on your account, so it is on your other devices and in Drafts & sent, which opens it again and can schedule it. If somebody else replies first, your words move into the thread they started. A list item’s comments begin its thread with the first comment.
New message offers what the conversation’s own box offers: user groups, @channel and @here where you may use them, K Buddy and apps’ commands — and to a public channel it lists the channel’s people first and says who is not in it. An apostrophe is any apostrophe in every people picker, and a deleted message leaves an open Unreads, the DMs list and Later at once. Leaving a channel, or being removed from it, stops replies in its threads from reaching you.