--- a/browser/base/content/tabview/tabitems.js
+++ b/browser/base/content/tabview/tabitems.js
@@ -1138,17 +1138,17 @@ let TabItems = {
   // ----------
   // Function: _checkHeartbeat
   // This periodically checks for tabs waiting to be updated, and calls
   // _update on them.
   // Should only be called by startHeartbeat and resumePainting.
   _checkHeartbeat: function TabItems__checkHeartbeat() {
     this._heartbeat = null;
 
-    if (this.isPaintingPaused() || !UI.isIdle)
+    if (this.isPaintingPaused() || !UI.isIdle())
       return;
 
     let accumTime = 0;
     let items = this._tabsWaitingForUpdate.getItems();
     // Do as many updates as we can fit into a "perceived" amount
     // of time, which is tunable.
     while (accumTime < this._maxTimeForUpdating && items.length) {
       let updateBegin = Date.now();

--- a/browser/base/content/test/tabview/Makefile.in
+++ b/browser/base/content/test/tabview/Makefile.in
@@ -125,16 +125,17 @@ include $(topsrcdir)/config/rules.mk
                  browser_tabview_bug634085.js \
                  browser_tabview_bug634158.js \
                  browser_tabview_bug634672.js \
                  browser_tabview_bug635696.js \
                  browser_tabview_bug640765.js \
                  browser_tabview_bug641802.js \
                  browser_tabview_bug644097.js \
                  browser_tabview_bug645653.js \
+                 browser_tabview_bug648882.js \
                  browser_tabview_dragdrop.js \
                  browser_tabview_exit_button.js \
                  browser_tabview_expander.js \
                  browser_tabview_firstrun_pref.js \
                  browser_tabview_group.js \
                  browser_tabview_launch.js \
                  browser_tabview_multiwindow_search.js \
                  browser_tabview_orphaned_tabs.js \

new file mode 100644
--- /dev/null
+++ b/browser/base/content/test/tabview/browser_tabview_bug648882.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  waitForExplicitFinish();
+
+  newWindowWithTabView(function (win) {
+    registerCleanupFunction(function () win.close());
+
+    let cw = win.TabView.getContentWindow();
+    let tab = win.gBrowser.tabs[0];
+    let tabItem = tab._tabViewTabItem;
+    let groupItem = cw.GroupItems.groupItems[0];
+    let container = groupItem.container;
+    let resizer = groupItem.$resizer[0];
+
+    let intervalID;
+    let isIdle = false;
+    let numLoops = 10;
+    let interval = cw.UI._maxInteractiveWait - 10;
+
+    let simulateDragDrop = function (target) {
+      EventUtils.synthesizeMouse(target, 5, 5, {type: "mousedown"}, cw);
+      EventUtils.synthesizeMouse(target, 40, 20, {type: "mousemove"}, cw);
+      EventUtils.synthesizeMouse(target, 20, 20, {type: "mouseup"}, cw);
+    }
+
+    let moveGroup = function () {
+      simulateDragDrop(container);
+
+      if (!--numLoops) {
+        numLoops = 10;
+        win.clearInterval(intervalID);
+        intervalID = win.setInterval(resizeGroup, interval);
+      }
+    };
+
+    let resizeGroup = function () {
+      simulateDragDrop(resizer);
+
+      if (!--numLoops) {
+        isIdle = true;
+        win.clearInterval(intervalID);
+      }
+    };
+
+    SimpleTest.waitForFocus(function () {
+      cw.TabItems.pausePainting();
+      cw.TabItems.update(tab);
+
+      tabItem.addSubscriber(tabItem, "updated", function () {
+        tabItem.removeSubscriber(tabItem, "updated");
+        ok(isIdle, "tabItem is updated only when UI is idle");
+        finish();
+      });
+
+      intervalID = win.setInterval(moveGroup, interval);
+      registerCleanupFunction(function () win.clearInterval(intervalID));
+
+      moveGroup();
+      cw.TabItems.resumePainting();
+    }, cw);
+  });
+}