diff --git a/static/js/message_viewport.js b/static/js/message_viewport.js
index 9543954d0a..dba3545008 100644
--- a/static/js/message_viewport.js
+++ b/static/js/message_viewport.js
@@ -9,6 +9,9 @@ var in_stoppable_autoscroll = false;
 // Includes both scroll and arrow events. Negative means scroll up,
 // positive means scroll down.
 exports.last_movement_direction = 1;
+exports.set_last_movement_direction = function (value) {
+    exports.last_movement_direction = value;
+};
 
 exports.at_top = function () {
     return exports.scrollTop() <= 0;
diff --git a/static/js/navigate.js b/static/js/navigate.js
index 90290918ef..db70d0ac5e 100644
--- a/static/js/navigate.js
+++ b/static/js/navigate.js
@@ -10,7 +10,7 @@ function go_to_row(msg_id) {
 }
 
 exports.up = function () {
-    message_viewport.last_movement_direction = -1;
+    message_viewport.set_last_movement_direction(-1);
     var msg_id = current_msg_list.prev();
     if (msg_id === undefined) {
         return;
@@ -19,7 +19,7 @@ exports.up = function () {
 };
 
 exports.down = function (with_centering) {
-    message_viewport.last_movement_direction = 1;
+    message_viewport.set_last_movement_direction(1);
 
     if (current_msg_list.is_at_end()) {
         if (with_centering) {
@@ -43,7 +43,7 @@ exports.down = function (with_centering) {
 };
 
 exports.to_home = function () {
-    message_viewport.last_movement_direction = -1;
+    message_viewport.set_last_movement_direction(-1);
     var first_id = current_msg_list.first().id;
     current_msg_list.select_id(first_id, {then_scroll: true,
                                           from_scroll: true});
@@ -51,7 +51,7 @@ exports.to_home = function () {
 
 exports.to_end = function () {
     var next_id = current_msg_list.last().id;
-    message_viewport.last_movement_direction = 1;
+    message_viewport.set_last_movement_direction(1);
     current_msg_list.select_id(next_id, {then_scroll: true,
                                          from_scroll: true});
     unread_ops.mark_current_list_as_read();
diff --git a/static/js/ui_init.js b/static/js/ui_init.js
index 4aab6244a4..009d17f425 100644
--- a/static/js/ui_init.js
+++ b/static/js/ui_init.js
@@ -70,7 +70,7 @@ exports.initialize_kitchen_sink_stuff = function () {
             }
         }
 
-        message_viewport.last_movement_direction = delta;
+        message_viewport.set_last_movement_direction(delta);
     }, 50);
 
     message_viewport.message_pane.on('wheel', function (e) {