diff --git a/zephyr/static/js/hotkey.js b/zephyr/static/js/hotkey.js
index 8fac78db69..1621aaff93 100644
--- a/zephyr/static/js/hotkey.js
+++ b/zephyr/static/js/hotkey.js
@@ -151,7 +151,7 @@ function process_hotkey(e) {
 
     switch (code) {
     case 33: // Page Up
-        if (at_top_of_viewport() && !current_msg_list.empty()) {
+        if (viewport.at_top() && !current_msg_list.empty()) {
             current_msg_list.select_id(current_msg_list.first().id, {then_scroll: false});
         }
         else {
@@ -160,7 +160,7 @@ function process_hotkey(e) {
         return true;
     case 32: // Spacebar
     case 34: // Page Down
-        if (at_bottom_of_viewport() && !current_msg_list.empty()) {
+        if (viewport.at_bottom() && !current_msg_list.empty()) {
             current_msg_list.select_id(current_msg_list.last().id, {then_scroll: false});
         }
         else {
diff --git a/zephyr/static/js/viewport.js b/zephyr/static/js/viewport.js
index c04d821f3c..c1e66b37fa 100644
--- a/zephyr/static/js/viewport.js
+++ b/zephyr/static/js/viewport.js
@@ -6,6 +6,18 @@ var height;
 var width;
 var in_stoppable_autoscroll = false;
 
+exports.at_top = function () {
+    return (jwindow.scrollTop() <= 0);
+};
+
+exports.at_bottom = function () {
+    // outerHeight(true): Include margin
+    var bottom = jwindow.scrollTop() + jwindow.height();
+    var window_size = $(document).height();
+
+    return bottom >= window_size;
+};
+
 exports.scrollTop = function viewport_scrollTop () {
     return jwindow.scrollTop.apply(jwindow, arguments);
 };
diff --git a/zephyr/static/js/zephyr.js b/zephyr/static/js/zephyr.js
index d219c3825f..1bd9f1d9d5 100644
--- a/zephyr/static/js/zephyr.js
+++ b/zephyr/static/js/zephyr.js
@@ -1120,15 +1120,6 @@ setInterval(function () {
     watchdog_time = new_time;
 }, 5000);
 
-function at_top_of_viewport() {
-    return (viewport.scrollTop() === 0);
-}
-
-function at_bottom_of_viewport() {
-    // outerHeight(true): Include margin
-    return (viewport.scrollTop() + viewport.height() >= $("#main_div").outerHeight(true));
-}
-
 function keep_pointer_in_view() {
     var candidate;
     var next_row = current_msg_list.selected_row();
@@ -1152,8 +1143,8 @@ function keep_pointer_in_view() {
         return true;
     }
 
-    if (! adjust(above_view_threshold, at_top_of_viewport, rows.next_visible))
-        adjust(below_view_threshold, at_bottom_of_viewport, rows.prev_visible);
+    if (! adjust(above_view_threshold, viewport.at_top, rows.next_visible))
+        adjust(below_view_threshold, viewport.at_bottom, rows.prev_visible);
 
     current_msg_list.select_id(rows.id(next_row), {from_scroll: true});
 }
@@ -1164,7 +1155,7 @@ function keep_pointer_in_view() {
 // 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(delta) {
-    if (delta !== 0 && (at_top_of_viewport() || at_bottom_of_viewport())) {
+    if (delta !== 0 && (viewport.at_top() || viewport.at_bottom())) {
         var next_row = current_msg_list.selected_row();
         if (delta > 0) {
             // Scrolling up (want older messages)