Save, then restore scroll position in Chrome textareas on remarkup assist

Summary:
Fixes T10396. Seems like this has been around for a while (references from 2011):

http://stackoverflow.com/questions/4002312/chrome-resets-the-textarea-scroll-bar-scrolltop-when-focus-is-called
https://bugs.chromium.org/p/chromium/issues/detail?id=75072

Commenting out this `focus()` seemed to fix the issue locally, at the cost of not focusing.

Saving, focusing, then restoring seems to produce the correct behavior everywhere.

Test Plan:
  - In Safari, Firefox and Chrome, typed a ton of text into a remarkup area (more than the height of the area, so it has a scrollbar).
  - Selected some text near the top.
  - Clicked "B" to bold the text.
  - Scroll position remained the same in all browsers (previously: in Chrome, it changed).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10396

Differential Revision: https://secure.phabricator.com/D15313

resources/celerity/map.php

@@ -8,7 +8,7 @@
 return array(
   'names' => array(
     'core.pkg.css' => '7935f211',
-    'core.pkg.js' => '298d5888',
+    'core.pkg.js' => '7d8faf57',
     'darkconsole.pkg.js' => 'e7393ebb',
     'differential.pkg.css' => '2de124c9',
     'differential.pkg.js' => 'd0cd0df6',
@@ -462,7 +462,7 @@ return array(
     'rsrc/js/core/Notification.js' => 'ccf1cbf8',
     'rsrc/js/core/Prefab.js' => 'e67df814',
     'rsrc/js/core/ShapedRequest.js' => '7cbe244b',
-    'rsrc/js/core/TextAreaUtils.js' => '9e54692d',
+    'rsrc/js/core/TextAreaUtils.js' => '5813016a',
     'rsrc/js/core/Title.js' => 'df5e11d2',
     'rsrc/js/core/ToolTip.js' => '6323f942',
     'rsrc/js/core/behavior-active-nav.js' => 'e379b58e',
@@ -775,7 +775,7 @@ return array(
     'phabricator-slowvote-css' => 'da0afb1b',
     'phabricator-source-code-view-css' => 'cbeef983',
     'phabricator-standard-page-view' => 'e709f6d0',
-    'phabricator-textareautils' => '9e54692d',
+    'phabricator-textareautils' => '5813016a',
     'phabricator-title' => 'df5e11d2',
     'phabricator-tooltip' => '6323f942',
     'phabricator-ui-example-css' => '528b19de',
@@ -1275,6 +1275,11 @@ return array(
       'javelin-request',
       'javelin-util',
     ),
+    '5813016a' => array(
+      'javelin-install',
+      'javelin-dom',
+      'javelin-vector',
+    ),
     '59a7976a' => array(
       'javelin-install',
       'javelin-dom',
@@ -1621,11 +1626,6 @@ return array(
       'phabricator-phtize',
       'changeset-view-manager',
     ),
-    '9e54692d' => array(
-      'javelin-install',
-      'javelin-dom',
-      'javelin-vector',
-    ),
     '9f36c42d' => array(
       'javelin-behavior',
       'javelin-stratcom',

webroot/rsrc/js/core/TextAreaUtils.js

@@ -33,7 +33,14 @@ JX.install('TextAreaUtils', {
 
     setSelectionRange : function(area, start, end) {
       if ('setSelectionRange' in area) {
+
+        // Chrome scrolls the textarea to the bottom as a side effect of
+        // calling focus(), so save the scroll position, focus, then restore
+        // the scroll position.
+        var scroll_top = area.scrollTop;
         area.focus();
+        area.scrollTop = scroll_top;
+
         area.setSelectionRange(start, end);
       }
     },