diff --git a/content/treestyletab/res/operationHistory.js b/content/treestyletab/res/operationHistory.js
index 164c90d0..d1fc88b3 100644
--- a/content/treestyletab/res/operationHistory.js
+++ b/content/treestyletab/res/operationHistory.js
@@ -6,10 +6,13 @@
 
    // window specific history
    OH.addEntry(
+     function() { // the task which should be undo-able.
+       MyService.myProp = newValue;
+     },
      'MyAddonFeature',
      { label  : 'Change tabbar position',
-       onUndo : function() { ... },
-       onRedo : function() { ... } },
+       onUndo : function() { MyService.myProp = oldValue; },
+       onRedo : function() { MyService.myProp = newValue; } },
      window
    );
    OH.undo('MyAddonFeature', window);
@@ -17,27 +20,31 @@
 
    // global history (not associated to window)
    OH.addEntry(
+     function() { ... }, // task
      'MyAddonFeature',
      { ... }
    );
    OH.undo('MyAddonFeature');
 
    // anonymous, window specific
-   OH.addEntry({ ... }, window);
+   OH.addEntry(function() { ... }, { ... }, window);
    OH.undo(window);
 
    // anonymous, global
-   OH.addEntry({ ... });
+   OH.addEntry(function() { ... }, { ... });
    OH.undo();
 
    // When you want to use "window" object in the global history,
    // you should use the ID string instead of the "window" object
    // to reduce memory leak. For example...
    OH.addEntry({
+     function() {
+       targetWindow.MyAddonService.myProp = newValue;
+     },
      id : OH.getWindowId(targetWindow),
      onUndo : function() {
        var w = OH.getWindowById(this.id);
-       w.MyAddonService.undoSomething();
+       w.MyAddonService.myProp = oldValue;
      },
      onRedo : ...
    });
@@ -56,7 +63,7 @@
    http://www.cozmixng.org/repos/piro/fx3-compatibility-lib/trunk/operationHistory.js
 */
 (function() {
-	const currentRevision = 3;
+	const currentRevision = 4;
 
 	if (!('piro.sakura.ne.jp' in window)) window['piro.sakura.ne.jp'] = {};
 
@@ -84,20 +91,37 @@
 
 		addEntry : function()
 		{
-			if (this._doingUndo)
-				return;
-
 			var options = this._getOptionsFromArguments(arguments);
-
 			var history = options.history;
 			var entries = history.entries;
+			var error;
+			var wasInUndoableTask = history._inUndoableTask;
 
-			entries = entries.slice(0, history.index+1);
-			entries.push(options.data);
-			entries = entries.slice(-this.kMAX_ENTRIES);
+			if (!wasInUndoableTask)
+				history._inUndoableTask = true;
 
-			history.entries = entries;
-			history.index = entries.length-1;
+			try {
+				if (options.task)
+					options.task.call(this);
+			}
+			catch(e) {
+				error = e;
+			}
+
+			if (!wasInUndoableTask && !this._doingUndo) {
+				entries = entries.slice(0, history.index+1);
+				entries.push(options.data);
+				entries = entries.slice(-this.kMAX_ENTRIES);
+
+				history.entries = entries;
+				history.index = entries.length-1;
+			}
+
+			if (!wasInUndoableTask)
+				delete history._inUndoableTask;
+
+			if (error)
+				throw e;
 		},
 
 		getHistory : function()
@@ -255,16 +279,18 @@
 
 		_getOptionsFromArguments : function(aArguments)
 		{
-			var w = null, name, data = null;
+			var w = null, name, data = null, task = null;
 			Array.slice(aArguments).some(function(aArg) {
 				if (aArg instanceof Ci.nsIDOMWindow)
 					w = aArg;
 				else if (typeof aArg == 'string')
 					name = aArg;
+				else if (typeof aArg == 'function')
+					task = aArg;
 				else if (aArg)
 					data = aArg;
 
-				return (w && name && data);
+				return (w && name && data && task);
 			});
 
 			if (!name)
@@ -289,7 +315,8 @@
 				window   : w,
 				windowId : windowId,
 				data     : data,
-				history  : this._tables[tableName]
+				history  : this._tables[tableName],
+				task     : task
 			};
 		},
 
diff --git a/content/treestyletab/treestyletabbrowser_tabpanelDNDObserver.js b/content/treestyletab/treestyletabbrowser_tabpanelDNDObserver.js
index c079cb56..26c65351 100644
--- a/content/treestyletab/treestyletabbrowser_tabpanelDNDObserver.js
+++ b/content/treestyletab/treestyletabbrowser_tabpanelDNDObserver.js
@@ -34,9 +34,12 @@ TreeStyleTabBrowserTabpanelDNDObserver.prototype = {
 				let orient = (position == 'left' || position == 'right') ? 'vertical' : 'horizontal' ;
 				sv.setTreePref('tabbar.fixed.'+orient, false);
 			}
-			var current = sv.currentTabbarPosition;
-			sv.currentTabbarPosition = position;
+			var current;
 			window['piro.sakura.ne.jp'].operationHistory.addEntry(
+				function() {
+					current = sv.currentTabbarPosition;
+					sv.currentTabbarPosition = position;
+				},
 				'TabbarDNDOperations',
 				{ label  : sv.treeBundle.getString('undo_changeTabbarPosition_label'),
 				  onUndo : function() { TreeStyleTabService.currentTabbarPosition = current; },
@@ -45,7 +48,10 @@ TreeStyleTabBrowserTabpanelDNDObserver.prototype = {
 			);
 		}
 
-		sv = null;
+		sv           = null;
+		aEvent       = null;
+		aXferData    = null;
+		aDragSession = null;
 
 		aEvent.stopPropagation();
 	},