Check if we need to rerender after appending messages to the MessageList

Previously, if the pointer was high on the page such that there was a
lot of empty space below and the render window was full (a situation
we could get into if we're following new messages arriving), a newly
recieved message would not be rendered even though it would have been
visible on the screen if we had done a rerender.  Adding the
_maybe_rerender() call to the end of append() ensures that we don't
end up in this situation.

(imported from commit 925d3cc62e8221b42f1d5ff1788e99c7d07ccc24)

zephyr/static/js/message_list.js

@@ -371,6 +371,13 @@ MessageList.prototype = {
             this._render(slice_to_render, 'bottom');
             this._render_win_end += slice_to_render.length;
         }
+
+        // If the pointer is high on the page such that there is a
+        // lot of empty space below and the render window is full, a
+        // newly recieved message should trigger a rerender so that
+        // the new message, which will appear in the viewable area,
+        // is rendered.
+        this._maybe_rerender();
     },
 
     prepend: function MessageList_prepend(messages) {