Fix issue where arrowing-down near end of page would skip a message.

We had this fascinating behavior where pressing a down arrow near the
end of the page would advance the pointer, call recenter_view, which
would trigger a scroll event, which would call keep_pointer_in_view,
which would notice that we were at the end of the page and advance the
pointer again!

I split out that last part into its own function which is only called
on mousewheel events.

(imported from commit bc85443e762356e3055f8f88585940a1f11f9124)

tools/jslint/check-all.js

@@ -39,7 +39,8 @@ var globals =
     // zephyr.js
     + ' message_array message_dict'
     + ' status_classes clear_table add_to_table instance_list'
-    + ' keep_pointer_in_view respond_to_message'
+    + ' keep_pointer_in_view move_pointer_at_page_top_and_bottom'
+    + ' respond_to_message'
     + ' select_message select_message_by_id'
     + ' scroll_to_selected select_and_show_by_id'
     + ' selected_message selected_message_id'

zephyr/static/js/ui.js

@@ -138,9 +138,15 @@ $(function () {
         sub_from_home(compose_class_name(), $('#class-nosub'));
     });
 
-    var throttled_scrollhandler = $.throttle(50, function() {
+    var throttled_scrollhandler = $.throttle(50, function(e) {
         if ($('#home').hasClass('active')) {
-                keep_pointer_in_view();
+            keep_pointer_in_view();
+            if (e.type === 'mousewheel') {
+                // If we mousewheel (or trackpad-scroll) when
+                // we're at the top and bottom of the page, the
+                // pointer may still want to move.
+                move_pointer_at_page_top_and_bottom();
+            }
         }
     });
     $(window).mousewheel(throttled_scrollhandler);

zephyr/static/js/zephyr.js

@@ -515,6 +515,16 @@ function keep_pointer_in_view() {
             }
         }
     }
+    update_selected_message(next_message);
+}
+
+// The idea here is when you've scrolled to the very
+// bottom of the page, e.g., the scroll handler isn't
+// going to fire anymore. But if I continue to use
+// the scrollwheel, the selection should advance until
+// I'm at the very top or the very bottom of the page.
+function move_pointer_at_page_top_and_bottom() {
+    var next_message = get_message_row(selected_message_id);
 
     if (at_top_of_viewport() && (parseInt(get_id(next_message), 10) >
                                  parseInt(get_id(get_first_visible()), 10))) {