Bug 1691144 - Trigger windows.onFocusChanged with no private window permission. r=zombie

Credits to Jonathan Mayer for submitting the original patch.
The patch changes the onFocusChanged extension event handler
so that it no longer bails out if focus is moving to a private
window, ensuring that the onFocusChanged event gets generated
with the proper WINDOW_ID_NONE identifier.

Differential Revision: https://phabricator.services.mozilla.com/D104732

browser/components/extensions/parent/ext-windows.js

@@ -92,10 +92,7 @@ this.windows = class extends ExtensionAPI {
               Promise.resolve().then(() => {
                 let windowId = Window.WINDOW_ID_NONE;
                 let window = Services.focus.activeWindow;
-                if (window) {
-                  if (!context.canAccessWindow(window)) {
-                    return;
-                  }
+                if (window && context.canAccessWindow(window)) {
                   windowId = windowTracker.getId(window);
                 }
                 if (windowId !== lastOnFocusChangedWindowId) {

browser/components/extensions/test/browser/browser_ext_windows_incognito.js

@@ -15,7 +15,22 @@ add_task(async function test_window_incognito() {
       permissions: ["http://mochi.test/"],
     },
     background() {
+      let lastFocusedWindowId = null;
+      // Catch focus change events to power the test below.
+      browser.windows.onFocusChanged.addListener(function listener(
+        eventWindowId
+      ) {
+        lastFocusedWindowId = eventWindowId;
+        browser.windows.onFocusChanged.removeListener(listener);
+      });
+
       browser.test.onMessage.addListener(async pbw => {
+        browser.test.assertEq(
+          browser.windows.WINDOW_ID_NONE,
+          lastFocusedWindowId,
+          "Focus on private window sends the event, but doesn't reveal windowId (without permissions)"
+        );
+
         await browser.test.assertRejects(
           browser.windows.get(pbw.windowId),
           /Invalid window ID/,
@@ -59,9 +74,13 @@ add_task(async function test_window_incognito() {
     },
   });
 
+  await extension.startup();
+
+  // The tests expect the incognito window to be
+  // created after the extension is started, so think
+  // carefully when moving this line.
   let winData = await getIncognitoWindow(url);
 
-  await extension.startup();
   extension.sendMessage(winData.details);
   await extension.awaitFinish("pass");
   await BrowserTestUtils.closeWindow(winData.win);