We now no longer do local echo if a user has logged in or visited a
narrow so recently that we are still fetching new messages for them in
their current message list.
Since we want any message list we're displaying to show only
contiguous sequences of messages within that view, it's not correct to
append messages that were just sent at the end unless
fetch_status.has_found_newest shows that we are up to date with the
latest messages from the server.
While we have some logic aimed at correcting our-of-order message IDs
in Zulip, even a brief (few seconds) temporary display of that is a
bug that we should avoid.
This means that we should disable local echo when the user's current
narrow is not up to date. We can be sure that we'll get the message
the user sent from the server either during the catch-up process or
when we receive it back from th server via the events system.
That particular race window can be several seconds in situations where
somebody is in a narrow where their pointer (or equivalent) is far
behind the latest messages.
This commit only fixes the local echo race condition. There's a
related bug where new messages sent by (potentially other) users
delivered to the client via server_events might race with our fetching
until we get the latest messages in a given narrow, which we'll need
to deal with separately.
See https://github.com/zulip/zulip/issues/8989 for more details. It's
possible that we'll close the issue after this fix, since any
additional fixes would add a lot of complexity, and I'm not sure how
much of a problem this will really be in practice after this fix.
Note that we don't have great automated testing for
`try_deliver_locally` (or really `echo.js` in general). For
`try_deliver_locally` the node tests would probably be 8x more complex
than the code itself, since that function is basically "glue" code
touching several external dependencies. It's also kind of hard to
screw up this code without getting pretty obvious failures early in
the QA process.
Fixes#8989.
6dba3f2420