Bug 828111 - Workaround multiple reflows issue and slow xbl attachment in the downloads view. r=mak

browser/components/downloads/content/allDownloadsViewOverlay.css

@@ -2,6 +2,21 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+/**
+ * The downloads richlistbox may list thousands of items, and it turns out
+ * XBL binding attachment, and even more so detachment, is a performance hog.
+ * This hack makes sure we don't apply any binding to inactive items (inactive
+ * items are history downloads that haven't been in the visible area).
+ * We can do this because the richlistbox implementation does not interact
+ * much with the richlistitem binding.  However, this may turn out to have
+ * some side effects (see bug 828111 for the details).
+ *
+ * We might be able to do away with this workaround once bug 653881 is fixed.
+ */
+richlistitem.download {
+  -moz-binding: none;
+}
+
 richlistitem.download[active] {
   -moz-binding: url('chrome://browser/content/downloads/download.xml#download-full-ui');
 }

browser/components/downloads/content/allDownloadsViewOverlay.js

@@ -1047,10 +1047,20 @@ DownloadsPlacesView.prototype = {
       }
     }
 
-    this._richlistbox.appendChild(elementsToAppendFragment);
+    this._appendDownloadsFragment(elementsToAppendFragment);
     this._ensureVisibleElementsAreActive();
   },
 
+  _appendDownloadsFragment: function DPV__appendDownloadsFragment(aDOMFragment) {
+    // Workaround multiple reflows hang by removing the richlistbox
+    // and adding it back when we're done.
+    let parentNode = this._richlistbox.parentNode;
+    let nextSibling = this._richlistbox.nextSibling;
+    parentNode.removeChild(this._richlistbox);
+    this._richlistbox.appendChild(aDOMFragment);
+    parentNode.insertBefore(this._richlistbox, nextSibling);
+  },
+
   nodeInserted: function DPV_nodeInserted(aParent, aPlacesNode) {
     this._addDownloadData(null, aPlacesNode);
   },