diff --git a/resources/celerity/map.php b/resources/celerity/map.php
--- a/resources/celerity/map.php
+++ b/resources/celerity/map.php
@@ -10,7 +10,7 @@
     'conpherence.pkg.css' => 'e68cf1fa',
     'conpherence.pkg.js' => '15191c65',
     'core.pkg.css' => '39061f68',
-    'core.pkg.js' => '1ea38af8',
+    'core.pkg.js' => 'e1f0f7bd',
     'differential.pkg.css' => '06dc617c',
     'differential.pkg.js' => 'c2ca903a',
     'diffusion.pkg.css' => 'a2d17c7d',
@@ -475,7 +475,7 @@
     'rsrc/js/core/behavior-more.js' => 'a80d0378',
     'rsrc/js/core/behavior-object-selector.js' => '77c1f0b0',
     'rsrc/js/core/behavior-oncopy.js' => '2926fff2',
-    'rsrc/js/core/behavior-phabricator-nav.js' => '836f966d',
+    'rsrc/js/core/behavior-phabricator-nav.js' => '94b7c320',
     'rsrc/js/core/behavior-phabricator-remarkup-assist.js' => 'acd29eee',
     'rsrc/js/core/behavior-read-only-warning.js' => 'ba158207',
     'rsrc/js/core/behavior-redirect.js' => '0213259f',
@@ -635,7 +635,7 @@
     'javelin-behavior-phabricator-keyboard-pager' => 'a8da01f0',
     'javelin-behavior-phabricator-keyboard-shortcuts' => '01fca1f0',
     'javelin-behavior-phabricator-line-linker' => '1e017314',
-    'javelin-behavior-phabricator-nav' => '836f966d',
+    'javelin-behavior-phabricator-nav' => '94b7c320',
     'javelin-behavior-phabricator-notification-example' => '8ce821c5',
     'javelin-behavior-phabricator-object-selector' => '77c1f0b0',
     'javelin-behavior-phabricator-oncopy' => '2926fff2',
@@ -1547,16 +1547,6 @@
       'javelin-behavior',
       'javelin-scrollbar',
     ),
-    '836f966d' => array(
-      'javelin-behavior',
-      'javelin-behavior-device',
-      'javelin-stratcom',
-      'javelin-dom',
-      'javelin-magical-init',
-      'javelin-vector',
-      'javelin-request',
-      'javelin-util',
-    ),
     '8499b6ab' => array(
       'javelin-behavior',
       'javelin-dom',
@@ -1658,6 +1648,16 @@
       'javelin-resource',
       'javelin-routable',
     ),
+    '94b7c320' => array(
+      'javelin-behavior',
+      'javelin-behavior-device',
+      'javelin-stratcom',
+      'javelin-dom',
+      'javelin-magical-init',
+      'javelin-vector',
+      'javelin-request',
+      'javelin-util',
+    ),
     '960f6a39' => array(
       'javelin-behavior',
       'javelin-dom',
diff --git a/webroot/rsrc/js/core/behavior-phabricator-nav.js b/webroot/rsrc/js/core/behavior-phabricator-nav.js
--- a/webroot/rsrc/js/core/behavior-phabricator-nav.js
+++ b/webroot/rsrc/js/core/behavior-phabricator-nav.js
@@ -49,7 +49,7 @@
       {
         element: drag,
         parameter: 'left',
-        start: JX.$V(drag).x
+        start: get_width()
       },
       {
         element: content,
@@ -102,15 +102,24 @@
       .setData(
         {
           key: 'filetree.width',
-          value: JX.$V(drag).x
+          value: get_width()
         })
       .send();
   });
 
+  function get_width() {
+    // See PHI568. If the document has scrolled horizontally, the "x" position
+    // of the bar will be the actual width of the menu plus the horizontal
+    // scroll position (because the element is "position: fixed"). Subtract the
+    // document scroll position when saving the element width so that scrolling
+    // to the right and then toggling the filetree UI does not make it grow
+    // any wider.
+    return (JX.$V(drag).x - JX.Vector.getScroll().x);
+  }
 
   var saved_width = config.width;
   function savedrag() {
-    saved_width = JX.$V(drag).x;
+    saved_width = get_width();
 
     local.style.width = '';
     drag.style.left = '';