Bug 486299: 'Back' button should be dimmed after clearing cache, r=gavin

mobile/chrome/content/browser.js

@@ -159,6 +159,7 @@ var Browser = {
     var os = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
     os.addObserver(gXPInstallObserver, "xpinstall-install-blocked", false);
     os.addObserver(gXPInstallObserver, "xpinstall-download-started", false);
+    os.addObserver(gSessionHistoryObserver, "browser:purge-session-history", false);
 
     // XXX hook up memory-pressure notification to clear out tab browsers
     //os.addObserver(function(subject, topic, data) self.destroyEarliestBrowser(), "memory-pressure", false);
@@ -231,6 +232,13 @@ var Browser = {
     }
   },
 
+  shutdown: function() {
+    var os = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
+    os.removeObserver(gXPInstallObserver, "xpinstall-install-blocked");
+    os.removeObserver(gXPInstallObserver, "xpinstall-download-started");
+    os.removeObserver(gSessionHistoryObserver, "browser:purge-session-history");
+  },
+
   updateViewportSize: function() {
     var [w, h] = this._canvasBrowser._effectiveContentAreaDimensions.map(Math.ceil);
 
@@ -1009,6 +1017,24 @@ const gXPInstallObserver = {
   }
 };
 
+const gSessionHistoryObserver = {
+  observe: function sho_observe(subject, topic, data) {
+    if (topic != "browser:purge-session-history")
+      return;
+
+    let back = document.getElementById("cmd_back");
+    back.setAttribute("disabled", "true");
+    let forward = document.getElementById("cmd_forward");
+    forward.setAttribute("disabled", "true");
+
+    let urlbar = document.getElementById("urlbar-edit");
+    if (urlbar) {
+      // Clear undo history of the URL bar
+      urlbar.editor.transactionManager.clear()
+    }
+  }
+};
+
 function getNotificationBox(aWindow) {
   return Browser.getNotificationBox();
 }

mobile/chrome/content/browser.xul

@@ -53,6 +53,7 @@
 
 <window id="main-window"
         onload="Browser.startup();"
+        onunload="Browser.shutdown();"
         windowtype="navigator:browser"
         title="&brandShortName;"
         titlemodifier="&brandShortName;"