Fix messages being added to the home view out of order.

So here's the reproduction recipe:
(1) Find a narrow that doesn't have any messages since 4 days ago

(2) Directly visit that narrow in your browser (or wait for someone to
do a deploy and thus auto-reload)

(3) Wait until load_old_messages has been called at least once

(4) Un-narrow

(5) Scroll up, and notice that the 400 most recent messages are above
sets of older messages.

The cause is that the code in add_messages assumed that
selected_message_id was within the range of message already in the
home view.  This is true in most cases where add_message is being
called, but it is not true in the case that the user was in a narrowed
view containing only very old messages (And thus selected_message_id
would be older than everything in the home view).

We can fix this by tracking persistent_message_id separately and using
that for the relevant test in add_messages.

(imported from commit f0da2561ba68f729343b260adc398029fae6acf7)

tools/jslint/check-all.js

@@ -69,7 +69,7 @@ var globals =
     + ' select_message select_message_by_id'
     + ' scroll_to_selected disable_pointer_movement get_private_message_recipient'
     + ' load_old_messages'
-    + ' selected_message selected_message_id'
+    + ' selected_message selected_message_id persistent_message_id'
     + ' at_top_of_viewport at_bottom_of_viewport'
     + ' viewport'
     + ' load_more_messages reset_load_more_status have_scrolled_away_from_top'

zephyr/static/js/narrow.js

@@ -2,9 +2,6 @@ var narrow = (function () {
 
 var exports = {};
 
-// For tracking where you were before you narrowed.
-var persistent_message_id = 0;
-
 // For narrowing based on a particular message
 var target_id = 0;
 
@@ -214,11 +211,6 @@ exports.activate = function (operators, opts) {
 
     allow_collapse          = opts.allow_collapse;
 
-    // Your pointer isn't changed when narrowed.
-    if (! was_narrowed) {
-        persistent_message_id = selected_message_id;
-    }
-
     // Before we clear the table, check if anything was highlighted.
     var highlighted = search.something_is_highlighted();
 

zephyr/static/js/zephyr.js

@@ -6,6 +6,9 @@ var people_dict = {};
 
 var viewport = $(window);
 
+// For tracking where you are in the home view
+var persistent_message_id = -1;
+
 var selected_message_id = -1;  /* to be filled in on document.ready */
 var selected_message = $();    /* = rows.get(selected_message_id)   */
 var get_updates_params = {
@@ -155,6 +158,7 @@ function send_pointer_update() {
 }
 
 $(function () {
+    persistent_message_id = initial_pointer;
     furthest_read = initial_pointer;
     server_furthest_read = initial_pointer;
     $(document).idle({idle: 1000,
@@ -168,13 +172,17 @@ function update_selected_message(message, opts) {
     message.addClass(cls);
 
     var new_selected_id = rows.id(message);
-    if (! narrow.active() && lurk_stream === undefined && new_selected_id > furthest_read)
-    {
-        // Narrowing is a temporary view on top of the home view and
-        // doesn't permanently affect where you are.
-        // Similarly, lurk mode does not affect your pointer.
-        furthest_read = new_selected_id;
+    // Narrowing is a temporary view on top of the home view and
+    // doesn't affect your pointer in the home view.
+    // Similarly, lurk mode does not affect your pointer.
+    if (! narrow.active() && lurk_stream === undefined) {
+        persistent_message_id = new_selected_id;
+        if (new_selected_id > furthest_read)
+        {
+            furthest_read = new_selected_id;
+        }
     }
+
     selected_message_id = new_selected_id;
     selected_message = message;
 }
@@ -484,11 +492,12 @@ function add_messages(messages, add_to_home) {
     messages = $.map(messages, add_message_metadata);
 
     if (add_to_home) {
+        // persistent_message_id is guaranteed to be between the top and bottom
         var top_messages_home = $.grep(messages, function (elem, idx) {
-            return (elem.id < selected_message_id && ! message_in_table.zhome[elem.id]);
+            return (elem.id < persistent_message_id && ! message_in_table.zhome[elem.id]);
         });
         var bottom_messages_home = $.grep(messages, function (elem, idx) {
-            return (elem.id >= selected_message_id && ! message_in_table.zhome[elem.id]);
+            return (elem.id >= persistent_message_id && ! message_in_table.zhome[elem.id]);
         });
         message_array = top_messages_home.concat(message_array).concat(bottom_messages_home);
         add_to_table(top_messages_home,    'zhome', narrow.in_home, "top",    true);
@@ -499,6 +508,7 @@ function add_messages(messages, add_to_home) {
     }
 
     if (narrow.active()) {
+        // selected_message_id is guaranteed to be between the top and bottom
         var top_messages_narrow = $.grep(messages, function (elem, idx) {
             return (elem.id < selected_message_id && ! message_in_table.zfilt[elem.id]);
         });