Avoid unnecessary fading/unfading processing via message_list.

The call to compose_fade.update_faded_messages() in message_list
caused us to traverse every message in the current table, which
was extremely inefficient.  Now we call the newly created
compose_fade.update_rendered_messages(), which only fades/unfades the
messages passed in as the first parameter.

(imported from commit a5a47e13fc9daeedd0899b2cfb02beb3f6b8cd0a)

static/js/compose_fade.js

@@ -110,6 +110,28 @@ exports.update_message_list = function () {
     }
 };
 
+exports.update_rendered_messages = function (messages, get_element) {
+    if (_want_normal_display()) {
+        return;
+    }
+
+    // This loop is superficially similar to some code in _fade_messages, but an
+    // important difference here is that we look at each message individually, whereas
+    // the other code takes advantage of blocks beneath recipient bars.
+    _.each(messages, function (message) {
+        var elt = get_element(message);
+        var should_fade_message = !util.same_recipient(focused_recipient, message);
+
+        if (should_fade_message) {
+            elt.removeClass("unfaded").addClass("faded");
+        } else {
+            elt.removeClass("faded").addClass("unfaded");
+        }
+    });
+
+    ui.update_floating_recipient_bar();
+};
+
 return exports;
 
 }());

static/js/message_list.js

@@ -621,8 +621,17 @@ MessageList.prototype = {
             this.select_id(this._selected_id, {from_rendering: true});
         }
 
-        // Re-add the fading of messages that is lost when we re-render.
-        compose_fade.update_faded_messages();
+        if (this === current_msg_list) {
+            // We don't have a Message class, but we can at least hide the messy details
+            // of rows.js from compose_fade.  We provide a callback function to be lazy--
+            // compose_fade may not actually need the elements depending on its internal
+            // state.
+            var get_element = function (message) {
+                return rows.get(message.id, table_name);
+            };
+
+            compose_fade.update_rendered_messages(messages, get_element);
+        }
 
         if (this === current_msg_list && messages_are_new) {
             this._maybe_autoscroll(rendered_elems);