Bug 1819675 - Introduce a feature pref to toggle the recently-closed tabs from all windows behavior.r=sclements,dao,extension-reviewers,fxview-reviewers,robwu,sessionstore-reviewers

* Add a default-true pref to provide an escape hatch allowing us to revert to previous behavior
* in which recently-closed tabs are per-window,
* and undoing closed tabs restores them to the window they were closed from.
* Ensure we set the pref for tests which depend on its value
* Add some spot-checks in tests with the pref off

Differential Revision: https://phabricator.services.mozilla.com/D179574

browser/components/sessionstore/SessionStore.sys.mjs

@@ -407,6 +407,11 @@ export var SessionStore = {
    * @param {Window} [aWindow] Optional window argument used to determine if we're counting for private or non-private windows
    */
   getClosedTabCount: function ss_getClosedTabCount(aWindow) {
+    if (!SessionStoreInternal._closedTabsFromAllWindowsEnabled) {
+      return this.getClosedTabCountForWindow(
+        aWindow ?? SessionStoreInternal._getTopWindow()
+      );
+    }
     return SessionStoreInternal.getClosedTabCount(aWindow);
   },
 
@@ -423,6 +428,11 @@ export var SessionStore = {
    * @param {Window} [aWindow] Optional window argument used to determine if we're collecting data for private or non-private windows
    */
   getClosedTabData: function ss_getClosedTabData(aWindow) {
+    if (!SessionStoreInternal._closedTabsFromAllWindowsEnabled) {
+      return this.getClosedTabDataForWindow(
+        aWindow ?? SessionStoreInternal._getTopWindow()
+      );
+    }
     return SessionStoreInternal.getClosedTabData(aWindow);
   },
 
@@ -1201,6 +1211,16 @@ var SessionStoreInternal = {
     );
     this._prefBranch.addObserver("sessionstore.max_tabs_undo", this, true);
 
+    this._closedTabsFromAllWindowsEnabled = this._prefBranch.getBoolPref(
+      "sessionstore.closedTabsFromAllWindows",
+      true
+    );
+    this._prefBranch.addObserver(
+      "sessionstore.closedTabsFromAllWindows",
+      this,
+      true
+    );
+
     this._max_windows_undo = this._prefBranch.getIntPref(
       "sessionstore.max_windows_undo"
     );
@@ -2699,6 +2719,12 @@ var SessionStoreInternal = {
           "sessionstore.restore_on_demand"
         );
         break;
+      case "sessionstore.closedTabsFromAllWindows":
+        this._closedTabsFromAllWindowsEnabled = this._prefBranch.getBoolPref(
+          "sessionstore.closedTabsFromAllWindows",
+          true
+        );
+        break;
     }
   },