Bug 1724960 - Add a one-time infobar to explain session restore. r=Gijs,fluent-reviewers

* Add a new browser.startup.couldRestoreSession.count which counts up (max 2) each time we *could* restore a session
* Show the new infobar notification when that count reaches 1 (i.e. skip it on the first startup after update)
* New strings for the notification - including an inline icon image for the ☰
* The notification's button opens the app menu.
* Allow Opt-out with a pref value of -1, and opt-out the testing user by default so our perf tests don't trip on it.

Differential Revision: https://phabricator.services.mozilla.com/D125904

browser/app/profile/firefox.js

@@ -299,6 +299,9 @@ pref("browser.startup.homepage.abouthome_cache.loglevel", "Warn");
 // Whether we should skip the homepage when opening the first-run page
 pref("browser.startup.firstrunSkipsHomepage", true);
 
+// Whether we should show the session-restore infobar on startup
+pref("browser.startup.couldRestoreSession.count", 0);
+
 // Show an about:blank window as early as possible for quick startup feedback.
 // Held to nightly on Linux due to bug 1450626.
 // Disabled on Mac because the bouncing dock icon already provides feedback.

browser/components/BrowserGlue.jsm

@@ -2389,6 +2389,7 @@ BrowserGlue.prototype = {
     }
 
     Sanitizer.onStartup();
+    this._maybeShowRestoreSessionInfoBar();
     this._scheduleStartupIdleTasks();
     this._lateTasksIdleObserver = (idleService, topic, data) => {
       if (topic == "idle") {
@@ -4177,6 +4178,73 @@ BrowserGlue.prototype = {
     });
   },
 
+  /**
+   * Only show the infobar when canRestoreLastSession and the pref value == 1
+   */
+  async _maybeShowRestoreSessionInfoBar() {
+    let count = Services.prefs.getIntPref(
+      "browser.startup.couldRestoreSession.count",
+      0
+    );
+    if (count < 0 || count >= 2) {
+      return;
+    }
+    if (count == 0) {
+      // We don't show the infobar right after the update which establishes this pref
+      // Increment the counter so we can consider it next time
+      Services.prefs.setIntPref(
+        "browser.startup.couldRestoreSession.count",
+        ++count
+      );
+      return;
+    }
+
+    // We've restarted at least once; we will show the notification if possible:
+    if (!SessionStore.canRestoreLastSession) {
+      return;
+    }
+
+    Services.prefs.setIntPref(
+      "browser.startup.couldRestoreSession.count",
+      ++count
+    );
+
+    const win = BrowserWindowTracker.getTopWindow();
+    const messageFragment = win.document.createDocumentFragment();
+    const message = win.document.createElement("span");
+    const icon = win.document.createElement("img");
+    icon.src = "chrome://browser/skin/menu.svg";
+    icon.setAttribute("data-l10n-name", "icon");
+    icon.className = "inline-icon";
+    message.appendChild(icon);
+    messageFragment.appendChild(message);
+    win.document.l10n.setAttributes(
+      message,
+      "restore-session-startup-suggestion-message"
+    );
+
+    const buttons = [
+      {
+        "l10n-id": "restore-session-startup-suggestion-button",
+        callback: () => {
+          win.PanelUI.show();
+        },
+      },
+    ];
+
+    const notifyBox = win.gBrowser.getNotificationBox();
+    const notification = notifyBox.appendNotification(
+      "startup-restore-session-suggestion",
+      {
+        label: messageFragment,
+        priority: notifyBox.PRIORITY_INFO_MEDIUM,
+      },
+      buttons
+    );
+    // Don't allow it to be immediately hidden:
+    notification.timeout = Date.now() + 3000;
+  },
+
   /**
    * Open preferences even if there are no open windows.
    */

browser/locales/en-US/browser/browser.ftl

@@ -814,3 +814,9 @@ tabs-toolbar-new-tab =
 tabs-toolbar-list-all-tabs =
     .label = List all tabs
     .tooltiptext = List all tabs
+
+## Infobar shown at startup to suggest session-restore
+
+# <img data-l10n-name="icon"/> will be replaced by the application menu icon
+restore-session-startup-suggestion-message = <strong>Open previous tabs?</strong> You can restore your previous session from the { -brand-short-name } application menu <img data-l10n-name="icon"/>, under History.
+restore-session-startup-suggestion-button = Show you how

testing/profiles/perf/user.js

@@ -34,6 +34,7 @@ user_pref("browser.safebrowsing.provider.google4.updateURL", "http://127.0.0.1/s
 user_pref("browser.safebrowsing.provider.mozilla.gethashURL", "http://127.0.0.1/safebrowsing-dummy/gethash");
 user_pref("browser.safebrowsing.provider.mozilla.updateURL", "http://127.0.0.1/safebrowsing-dummy/update");
 user_pref("browser.shell.checkDefaultBrowser", false);
+user_pref("browser.startup.couldRestoreSession.count", -1);
 user_pref("browser.tabs.remote.autostart", true);
 user_pref("browser.warnOnQuit", false);
 user_pref("datareporting.healthreport.documentServerURI", "http://127.0.0.1/healthreport/");

toolkit/content/widgets/message-bar.css

@@ -216,6 +216,11 @@
   vertical-align: middle;
 }
 
+/* Align inline icon images in the message content */
+.container.infobar > .notification-content > .notification-message img.inline-icon {
+  vertical-align: bottom;
+}
+
 .close {
   margin: 4px 8px;
   background-size: 16px;