msglist/compose: In interleaved views, make it easier to specify "destination narrow" for a reply

[chrisbobbe]

Contrast with #493, which is about anticipating the right destination, which is different from letting the user specify it.

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Users sometimes want to reply directly from an interleaved view, like a whole-stream view or the "All messages" view.

It should be easier to set the compose box to a desired destination narrow, for example by filling the topic input.

A user sent a support email explaining that, in zulip-mobile, it seems too effortful to specify a topic in an interleaved stream view. In a stream view in zulip-mobile, the topic input starts out as empty. If you read a message and you want to reply to it, you have these options:

• Type out the topic in the topic input, using autocomplete if it's helpful. If you used autocomplete, the message input will auto-focus for you; otherwise, tap it to focus.
• Long-press the message and tap "Reply", which takes you to the single-topic narrow. Tap the message input to focus it.
• Long-press the message and tap "Quote and reply", which (since v27.212 in 2023-10) will fill in the topic, fill in the message input by quoting the selected message, and focus the message input. If you don't want the quote, you have to select and delete it.

We can offer a more streamlined way to specify the destination narrow in interleaved views. (For replying to a stream message, this means pre-filling the topic input. For a DM, it might mean pre-filling a hypothetical input for the recipients.) How about:

• If you double-tap a message, we choose the destination narrow from that message. (This was suggested by the user who emailed us.) Or:
• If you single-tap a message, we choose the destination narrow from that message. (As in an old zulip-mobile proposal: Tapping on a message should begin a Reply. zulip-mobile#2901).

We should be careful to not let the action (double- or single-tapping) reset the user's compose session if they don't want it to. If the user has already chosen a destination and they're writing a message, and then they double-tap a message in the message list, we should ask before clearing out the compose box.

[chrisbobbe]

As with #493, if we don't think we'll ever have a compose box that lets you select DM recipients, we might narrow this issue's scope so that we're only helpful in the case of stream-message replies. Questions about how DM replies work shouldn't prevent us from being helpful in whole-stream views, anyway, even if we can't easily settle how "All messages" should work.

[gnprice]

Related issue:

• Review UX of compose box in interleaved narrows #149