If there's an anchor in the URL in Differential, don't stick to the bottom of the page as content loads

Summary:
Fixes T11784. A lot of things are interacting here, but this probably gets slightly better results slightly more often?

Basically:

  - When we load content, we try to keep the viewport "stable" on the page, so the page doesn't jump around like crazy.
  - If you're near the top or bottom of the page, we try to stick to the top (e.g., reading the summary) or bottom (e.g., writing a comment).
  - But, if you followed an anchor to a comment that's close to the bottom of the page, we might stick to the bottom intead of staying with the anchor.

Kind of do a better job by not sticking to the bottom if you have an anchor. This will get things wrong if you follow an anchor, scroll down, start writing a comment, etc. But this whole thing is a pile of guesses anyway.

Test Plan:
  - Followed an anchor, saw non-sticky stabilization.
  - Loaded the page normally, scrolled to the bottom real fast, saw sticky stabilization.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11784

Differential Revision: https://secure.phabricator.com/D17911

resources/celerity/map.php

@@ -13,7 +13,7 @@ return array(
     'core.pkg.js' => '8c5f913d',
     'darkconsole.pkg.js' => '1f9a31bc',
     'differential.pkg.css' => '58712637',
-    'differential.pkg.js' => 'bd321b6e',
+    'differential.pkg.js' => 'f1b636fb',
     'diffusion.pkg.css' => 'b93d9b8c',
     'diffusion.pkg.js' => '84c8f8fd',
     'favicon.ico' => '30672e08',
@@ -390,7 +390,7 @@ return array(
     'rsrc/js/application/dashboard/behavior-dashboard-move-panels.js' => '408bf173',
     'rsrc/js/application/dashboard/behavior-dashboard-query-panel-select.js' => '453c5375',
     'rsrc/js/application/dashboard/behavior-dashboard-tab-panel.js' => 'd4eecc63',
-    'rsrc/js/application/diff/DiffChangeset.js' => '145c34e2',
+    'rsrc/js/application/diff/DiffChangeset.js' => 'c5742feb',
     'rsrc/js/application/diff/DiffChangesetList.js' => 'f1101e6e',
     'rsrc/js/application/diff/DiffInline.js' => 'bdf6b568',
     'rsrc/js/application/diff/behavior-preview-link.js' => '051c7832',
@@ -779,7 +779,7 @@ return array(
     'phabricator-darklog' => 'c8e1ffe3',
     'phabricator-darkmessage' => 'c48cccdd',
     'phabricator-dashboard-css' => 'fe5b1869',
-    'phabricator-diff-changeset' => '145c34e2',
+    'phabricator-diff-changeset' => 'c5742feb',
     'phabricator-diff-changeset-list' => 'f1101e6e',
     'phabricator-diff-inline' => 'bdf6b568',
     'phabricator-drag-and-drop-file-upload' => '58dea2fa',
@@ -997,17 +997,6 @@ return array(
       'javelin-dom',
       'javelin-typeahead-normalizer',
     ),
-    '145c34e2' => array(
-      'javelin-dom',
-      'javelin-util',
-      'javelin-stratcom',
-      'javelin-install',
-      'javelin-workflow',
-      'javelin-router',
-      'javelin-behavior-device',
-      'javelin-vector',
-      'phabricator-diff-inline',
-    ),
     '1499a8cb' => array(
       'javelin-behavior',
       'javelin-stratcom',
@@ -1926,6 +1915,17 @@ return array(
       'javelin-stratcom',
       'phabricator-tooltip',
     ),
+    'c5742feb' => array(
+      'javelin-dom',
+      'javelin-util',
+      'javelin-stratcom',
+      'javelin-install',
+      'javelin-workflow',
+      'javelin-router',
+      'javelin-behavior-device',
+      'javelin-vector',
+      'phabricator-diff-inline',
+    ),
     'c587b80f' => array(
       'javelin-install',
     ),

webroot/rsrc/js/application/diff/DiffChangeset.js

@@ -440,6 +440,12 @@ JX.install('DiffChangeset', {
       var near_top = (old_pos.y <= sticky);
       var near_bot = ((old_pos.y + old_view.y) >= (old_dim.y - sticky));
 
+      // If we have an anchor in the URL, never stick to the bottom of the
+      // page. See T11784 for discussion.
+      if (window.location.hash) {
+        near_bot = false;
+      }
+
       var target_pos = JX.Vector.getPos(target);
       var target_dim = JX.Vector.getDim(target);
       var target_mid = (target_pos.y + (target_dim.y / 2));