Make waving cover files around on boards more reliable

Summary:
Currently, in Safari, if you drag an image onto a board to make it a cover file and then wave it around wildly a lot over differnent cards, it sometimes glitches out a bit and won't drop on them properly.

This appears to be because sequencing and delivery of dragenter/dragleave events isn't always totally ideal.

Instead, just cancel any existing drag when we get a new drag that targets a new drop target.

Test Plan:
  - Opened a board with a bunch of cards.
  - Dragged a file from my desktop onto the board.
  - Waved it around wildly, hovering over many different cards.
    - Before patch: sometimes cards under the cursor stopped highlighting properly.
    - After patch: behavior seems correct and consistent.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15208

resources/celerity/map.php

@@ -11,7 +11,7 @@ return array(
     'core.pkg.js' => '808ae845',
     'darkconsole.pkg.js' => 'e7393ebb',
     'differential.pkg.css' => '2de124c9',
-    'differential.pkg.js' => '6b42b4bc',
+    'differential.pkg.js' => 'd0cd0df6',
     'diffusion.pkg.css' => 'f45955ed',
     'diffusion.pkg.js' => '3a9a8bfa',
     'maniphest.pkg.css' => '4845691a',
@@ -446,7 +446,7 @@ return array(
     'rsrc/js/application/uiexample/gesture-example.js' => '558829c2',
     'rsrc/js/application/uiexample/notification-example.js' => '8ce821c5',
     'rsrc/js/core/Busy.js' => '59a7976a',
-    'rsrc/js/core/DragAndDropFileUpload.js' => 'da044194',
+    'rsrc/js/core/DragAndDropFileUpload.js' => '81f182b5',
     'rsrc/js/core/DraggableList.js' => '8905523d',
     'rsrc/js/core/FileUpload.js' => '680ea2c8',
     'rsrc/js/core/Hovercard.js' => '1bd28176',
@@ -741,7 +741,7 @@ return array(
     'phabricator-core-css' => '5b3563c8',
     'phabricator-countdown-css' => 'e7544472',
     'phabricator-dashboard-css' => 'eb458607',
-    'phabricator-drag-and-drop-file-upload' => 'da044194',
+    'phabricator-drag-and-drop-file-upload' => '81f182b5',
     'phabricator-draggable-list' => '8905523d',
     'phabricator-fatal-config-template-css' => '8e6c6fcd',
     'phabricator-feed-css' => 'ecd4ec57',
@@ -1451,6 +1451,14 @@ return array(
       'javelin-vector',
       'javelin-stratcom',
     ),
+    '81f182b5' => array(
+      'javelin-install',
+      'javelin-util',
+      'javelin-request',
+      'javelin-dom',
+      'javelin-uri',
+      'phabricator-file-upload',
+    ),
     '834a1173' => array(
       'javelin-behavior',
       'javelin-scrollbar',
@@ -1890,14 +1898,6 @@ return array(
       'javelin-util',
       'phabricator-shaped-request',
     ),
-    'da044194' => array(
-      'javelin-install',
-      'javelin-util',
-      'javelin-request',
-      'javelin-dom',
-      'javelin-uri',
-      'phabricator-file-upload',
-    ),
     'dbbf48b6' => array(
       'javelin-behavior',
       'javelin-stratcom',

webroot/rsrc/js/core/DragAndDropFileUpload.js

@@ -107,13 +107,18 @@ JX.install('PhabricatorDragAndDropFileUpload', {
           return;
         }
 
-        if (!this._node && !this._depth) {
+        if (!this._node) {
-          this._target = e.getNode(this._sigil);
+          var target = e.getNode(this._sigil);
+          if (target !== this._target) {
+            this._updateDepth(-this._depth);
+            this._target = target;
+          }
         }
 
         if (contains(this._getTarget(), e.getTarget())) {
           this._updateDepth(1);
         }
+
       });
 
       var on_dragleave = JX.bind(this, function(e) {
@@ -121,6 +126,10 @@ JX.install('PhabricatorDragAndDropFileUpload', {
           return;
         }
 
+        if (!this._getTarget()) {
+          return;
+        }
+
         if (contains(this._getTarget(), e.getTarget())) {
           this._updateDepth(-1);
         }