msglist: Fetch-older is defeated by a run of 100+ muted messages

[chrisbobbe]

I ran into an odd bug yesterday in the iOS app:

1. Muted a topic ("zulip.yaml") in the #api documentation channel on CZO
2. In the Flutter app, opened the channel narrow for #api documentation

Expected: to see the channel feed as usual, just without messages in the topic I muted
Actual: I only saw one message! 🙂 The most recent in the channel. And nothing above it saying I'd reached the start of the channel, or that it was still loading more messages:

Diagnosis

I think two things are breaking this:

• When the visible messages occupy much less than a screenful, the ScrollPhysics doesn't allow changing the scroll position, so the user can't trigger a needed fetchOlder. This is happening here because apparently 99 of the 100 messages from fetchInitial are muted. A simple fix is to use AlwaysScrollableScrollPhysics for the message list.
• Normally, a fetchOlder response will cause us to do the next fetchOlder request with a different, older message as the "anchor". Here, though, we're sending the same fetchOlder request with the same anchor. Why? Because the anchor we request is the oldest visible (non-muted; _messageVisible) message. If the fetchOlder response is entirely messages that we don't count as visible, then "the oldest visible message", and therefore the anchor, won't change.

[gnprice]

A report today of what looks like this issue:
#mobile > Empty All messages view @ 💬

I'll put this in the M6 Post-launch milestone (I think this doesn't need to block the app's launch), but sorted to the earlier part of that milestone on the project board.

[timabbott]

Just a small note on how we handle this class of issue in the web app: We store for each message list an all_items (all messages matching its filters, including muted messages even if they won't be shown) and an items (that set filtered by muting). The main purpose of that arrangement is to allow us to live-update the data structures if a topic that matched this view gets unmuted, but it's also used for setting these anchors based on the all_items data, not the items data.