zulip/docs/subsystems/pointer.md

Unread counts and the pointer

When you're using Zulip and you reload, or narrow to a stream, how does Zulip decide where to place you?

Conceptually, Zulip takes you to the place where you left off (e.g. the first unread message), not the most recent messages, to facilitate reviewing all the discussions that happened while you were away from your computer. The scroll position is then set to keep that message in view and away from both the top and bottom of the visible section of messages.

But there a lot of details around doing this right, and around counting unread messages. Here's how Zulip currently decides which message to select, along with some notes on improvements we'd like to make to the model.

First a bit of terminology:

• "Narrowing" is the process of filtering to a particular subset of the messages the user has access to.

• The blue cursor box (the "pointer") is around is called the "selected" message. Zulip ensures that the currently selected message is always in-view.

Pointer logic

Recipient bar: message you clicked

If you enter a narrow by clicking on a message group's recipient bar (stream/topic or private message recipient list at the top of a group of messages), Zulip will select the message you clicked on. This provides a nice user experience where you get to see the stuff near what you clicked on, and in fact the message you clicked on stays at exactly the same scroll position in the window after the narrowing as it was at before.

Search, sidebar click, or new tab: unread/recent matching narrow

If you instead narrow by clicking on something in the left sidebar, typing some terms into the search box, reloading the browser, or any other method that doesn't encode a specific message to visit, Zulip will instead select the first unread message matching that narrow, or if there are none, the most recent messages matching that narrow.

This provides the nice user experience of taking you to the start of the new stuff (with enough messages you'ev seen before still in view at the top to provide you with context), which is usually what you want. (When finding the "first unread message", Zulip ignores unread messages in muted streams or in muted topics within non-muted streams.)

Unnarrow: previous sequence

When you unnarrow using e.g. the a key, you will automatically be taken to the same message that was selected in the All messages view before you narrowed, unless in the narrow you read new messages, in which case you will be jumped forward to the first unread and non-muted message in the All messages view (or the bottom of the feed if there is none). This makes for a nice experience reading threads via the All messages view in sequence.

Forced reload: state preservation

When the server forces a reload of a browser that's otherwise caught up (which happens within 30 minutes when a new version of the server is deployed, usually at a type when the user isn't looking at the browser), Zulip will preserve the state -- what (if any) narrow the user was in, the selected message, and even exact scroll position!

For more on the user experience philosophy guiding these decisions, see the architectural overview.

Unread count logic

How does Zulip decide whether a message has been read by the user? The algorithm needs to correctly handle a range of ways people might use the product. The algorithm is as follows:

• Any message which is selected or above a message which is selected is marked as read. So messages are marked as read as you scroll down the keyboard when the pointer passes over them.

• If the whitespace at the very bottom of the feed is in view, all messages in view are marked as read.

These two simple rules, combined with the pointer logic above, end up matching user expectations well for whether the product should treat them as having read a set of messages (or not).

One key detail to highlight is that we only mark messages as read through these processes in views that contain all messages in a thread; search views will never mark messages as read.

Testing and development

In a Zulip development environment, you can use manage.py mark_all_messages_unread to set every user's pointer to 0 and all messages as unread, for convenience in testing unread count related logic.

It can be useful to combine this with manage.py populate_db -n 3000 (which rebuilds the database with 3000 initial messages) to ensure a large number of messages are present.