Expand pointer and unread counts guide.

docs/pointer.md

@@ -1,40 +1,54 @@
-# Pointer and unread counts
+# 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?
 
-In general, 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.
+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 decides which message to
-select.
+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.
 
-## Recipient bar: message you clicked
+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 center the new narrow around the message you
-clicked on. This provides a nice user experience where you get to see
-the stuff near what you clicked on.
+of messages), Zulip will select the 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 or sidebar click: unread/recent matching narrow
+### Search or sidebar click: unread/recent matching narrow
 
 If you instead narrow by clicking on something in the left sidebar or
-typing some terms into the search box, Zulip will instead focus on 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 maybe a
-bit of old stuff still in view at the top), 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.)
+typing some terms into the search box, Zulip will instead selected on
+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
+### Unnarrow: previous sequence
 
 When you unnarrow using e.g. the escape key, you will automatically be
 taken to the same message that was selected in the home view before
@@ -44,32 +58,53 @@ message in the home view (or the bottom of the feed if there is
 none). This makes for a nice experience reading threads via the home
 view in sequence.
 
-## New home view: "high watermark"
+### New home view: "high watermark"
 
 When you open a new browser window or tab to the home view (a.k.a. the
-interleaved view you get if you visit `/`), Zulip will take you to the
-highest message ID (a.k.a. "high watermark") that your cursor has ever
-reached in the home view (called the *"pointer"* in the Zulip
-API). Because of the logic around unnarrowing in the last bullet, this
-is usually the same as the first unread message in the home view, but
+interleaved view you get if you visit `/`), Zulip will select the
+furthest down that your cursor has ever reached in the home
+view. Because of the logic around unnarrowing in the last bullet, this
+is usually just before the first unread message in the home view, but
 if you never go to the home view, or you leave messages unread on some
 streams in your home view, this can lag.
 
-## Narrow in a new tab: closest to pointer
+We plan to change this to automatically advance the pointer in a way
+similar to the unnarrow logic.
+
+### Narrow in a new tab: closest to pointer
 
 When you load a new browser tab or window to a narrowed view, Zulip
-will take you to the message closest to your pointer, which is what
-you would have got had you loaded the browser window to your home view
-and then clicked on the nearest message matching your narrow (which
-might have been offscreen).
+will select the message closest to your pointer, which is what you
+would have got had you loaded the browser window to your home view and
+then clicked on the nearest message matching your narrow (which might
+have been offscreen).
 
-## Forced reload: historical recreation
+We plan to change this to match the Search/sidebar behavior.
+
+### 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), Zulip will try to take the user back to the exact same
-place where you were before the server-initiated reload, in every way
-(same selected message, and even the same exact scroll position!).
+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](architecture-overview.html).
+
+## 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 whitspace 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).