fix(addon): #808 fix uncertain load_reason for the back button

lib/TabTracker.js

@@ -174,8 +174,7 @@ TabTracker.prototype = {
     // already ended, likely with an 'unfocus' unload reason.
     if (this.isActivityStreamsURL(tab.url) && tabs.activeTab.id === tab.id) {
       if (!this._tabData.url) {
-        // load_reason could have been set as 'back_button' from before.
-        this._tabData.load_reason = this._tabData.load_reason || "newtab";
+        this._tabData.load_reason = "newtab";
       } else if (!this._tabData.session_duration) {
         // The page content has been reloaded but a total time wasn't set.
         // This is due to a page refresh. Let's set the total time now.
@@ -202,7 +201,9 @@ TabTracker.prototype = {
     // Thus, if we get here and load_reason still has not been set, then we know
     // we got here due to a click of the 'back' button.
     if (this.isActivityStreamsURL(tab.url) && !this._tabData.load_reason) {
-      this._tabData.load_reason = "back_button";
+      // logReady will start a new session and set the 'load_reason' as 'newtab'.
+      // we do not use 'back_button' for the 'load_reason' due to a known issue:
+      // https://github.com/mozilla/activity-stream/issues/808
       this.logReady(tab);
     }
   },

test/test-TabTracker.js

@@ -224,7 +224,7 @@ exports.test_TabTracker_back_button_load = function*(assert) {
   yield tabClosedPromise;
   yield pingsSentPromise;
 
-  let loadReasons = ["newtab", "back_button"];
+  let loadReasons = ["newtab", "newtab"];
   let unloadReasons = ["refresh", "close"];
   checkLoadUnloadReasons(assert, pingData, loadReasons, unloadReasons);
 };