Zulip: Narrowing to a topic with compose open should prefill the topic input box

[strugee]

I find that frequently I'll be looking at a stream where all the most recent messages are from one topic, so I think I'm just narrowed to that topic. I open the compose box and find that the topic _isn't_ prefilled (because I'm only narrowed to a stream), so I click on the topic's name to fix the narrowing for the topic to autofill, but this closes the compose box.

It'd be a nice usability enhancement if the compose box remained open in such a case and the topic was filled out to match the new narrowing parameters.

[timabbott]

We'll need to be careful with this, since it's easy to prefill things in a way that causes users to accidentally send to the wrong place, but I think that in this specific flow (you're narrowed to a stream, have an open but empty compose box, with no topic set, and then narrow to a topic) seems pretty safe.

[kracekumar]

I came to file the same issue and found the existing issue. So :+1:

[timabbott]

@kracekumar if you want to work on this, exports.activate in narrow.js is the code path in question.

Actually @showell this is potentially worth fixing while we're at it working on sidebar narrowing.