Bug 1874599 - Add a pref to disable userContextId guessing for external opens;r=mossop

Bug 1692124 improved the behavior for workflows where external opens should move into sessions
inside containers, but it created an issue for those where external opens should move into
the default container and there are also tabs from the same domains inside containers, and
would create extra interstitials with the Multi-Account Containers addon if the predicted
container is different than the "always open" addon setting.

While the heuristic could be improved, it will never be perfect for all workflows,
so this patch introduces a pref to opt out of the behavior (and always open external
links in the default user context ID).

Differential Revision: https://phabricator.services.mozilla.com/D199443

Author: bgrins

browser/app/profile/firefox.js

@@ -826,6 +826,10 @@ pref("permissions.desktop-notification.notNow.enabled", false);
 
 pref("permissions.fullscreen.allowed", false);
 
+// Force external link opens into the default user context ID instead of guessing
+// the most appropriate one based on the URL (https://bugzilla.mozilla.org/show_bug.cgi?id=1874599#c8)
+pref("browser.link.force_default_user_context_id_for_external_opens", false);
+
 // handle links targeting new windows
 // 1=current window/tab, 2=new window, 3=new tab in most recent window
 pref("browser.link.open_newwindow", 3);

browser/base/content/browser.js

@@ -6139,8 +6139,15 @@ nsBrowserAccess.prototype = {
   ) {
     var browsingContext = null;
     var isExternal = !!(aFlags & Ci.nsIBrowserDOMWindow.OPEN_EXTERNAL);
+    var guessUserContextIdEnabled =
+      isExternal &&
+      !Services.prefs.getBoolPref(
+        "browser.link.force_default_user_context_id_for_external_opens",
+        false
+      );
     var openingUserContextId =
-      (isExternal && URILoadingHelper.guessUserContextId(aURI)) ||
+      (guessUserContextIdEnabled &&
+        URILoadingHelper.guessUserContextId(aURI)) ||
       Ci.nsIScriptSecurityManager.DEFAULT_USER_CONTEXT_ID;
 
     if (aOpenWindowInfo && isExternal) {

browser/components/contextualidentity/test/browser/browser_guessusercontext.js

@@ -27,6 +27,9 @@ async function openTabInUserContext(uri, userContextId, win = window) {
 }
 
 registerCleanupFunction(async function cleanup() {
+  Services.prefs.clearUserPref(
+    "browser.link.force_default_user_context_id_for_external_opens"
+  );
   while (gBrowser.tabs.length > 1) {
     gBrowser.removeTab(gBrowser.selectedTab, { animate: false });
   }
@@ -69,19 +72,7 @@ add_task(async function test() {
   is(guessUserContextId(HOST_EXAMPLE), WORK, "forgets closed window");
 
   // Check the opener flow more directly
-  let browsingContext = window.browserDOMWindow.openURI(
-    makeURI(HOST_EXAMPLE.spec + "?new"),
-    null,
-    Ci.nsIBrowserDOMWindow.OPEN_NEWTAB,
-    Ci.nsIBrowserDOMWindow.OPEN_EXTERNAL,
-    Services.scriptSecurityManager.getSystemPrincipal()
-  );
-  await BrowserTestUtils.browserLoaded(browsingContext.embedderElement);
-  is(
-    browsingContext.embedderElement,
-    gBrowser.selectedBrowser,
-    "opener selected"
-  );
+  openURIFromExternal(HOST_EXAMPLE.spec + "?new");
   is(
     gBrowser.selectedTab.getAttribute("usercontextid"),
     WORK.toString(),
@@ -93,4 +84,32 @@ add_task(async function test() {
     DEFAULT,
     "still matches default container"
   );
+
+  // Force into default with the pref from https://bugzilla.mozilla.org/show_bug.cgi?id=1692124
+  Services.prefs.setBoolPref(
+    "browser.link.force_default_user_context_id_for_external_opens",
+    true
+  );
+  openURIFromExternal(HOST_EXAMPLE.spec + "?new");
+  is(
+    gBrowser.selectedTab.getAttribute("usercontextid"),
+    "",
+    "opener flow with default user context ID forced by pref"
+  );
 });
+
+async function openURIFromExternal(spec) {
+  let browsingContext = window.browserDOMWindow.openURI(
+    makeURI(spec),
+    null,
+    Ci.nsIBrowserDOMWindow.OPEN_NEWTAB,
+    Ci.nsIBrowserDOMWindow.OPEN_EXTERNAL,
+    Services.scriptSecurityManager.getSystemPrincipal()
+  );
+  await BrowserTestUtils.browserLoaded(browsingContext.embedderElement);
+  is(
+    browsingContext.embedderElement,
+    gBrowser.selectedBrowser,
+    "opener selected"
+  );
+}