Bug 1888310: Verify a11y-announcement is a child of ChildView instead of mozRootAccessible r=eeejay

Previously we checked if the announcement was a child of the root acc directly, but it's not clear this worked reliably since we've done `GetObjectOrRepresentedView` for a while, which (should) return the view instead of the root acc. This patch also:
- Dispatches the announcement from NSApp instead of NSWindow, since per chrome and safari notifs fired on non-main windows get dropped
- Modifies the announcement priority from medium to high, so VO interrupts itself to speak this message (this makes the UX more consistent, since the text-inserted/text-deleted notifs from the URL bar seem to occasionally bookend the announcement)
- Updates the browser_app.js test to NOT run in headless mode, since in headless mode NSApp isn't rendered to dispatch the notification. This test contains a task for AXAnnouncementRequested via a11yUtils.announce

Differential Revision: https://phabricator.services.mozilla.com/D206083

accessible/mac/mozAccessible.mm

@@ -879,10 +879,13 @@ struct RoleDescrComparator {
   // a random acc with the same ID) by checking:
   //  - The gecko acc is local, our a11y-announcement lives in browser.xhtml
   //  - The ID of the gecko acc is "a11y-announcement"
-  //  - The native acc is a direct descendent of the root
+  //  - The native acc is a direct descendent of the chrome window (ChildView in
+  //  a non-headless context, mozRootAccessible in a headless context).
   if (mGeckoAccessible->IsLocal() &&
       [[self moxDOMIdentifier] isEqualToString:@"a11y-announcement"] &&
-      [[self moxParent] isKindOfClass:[mozRootAccessible class]]) {
+      ((gfxPlatform::IsHeadless() &&
+        [[self moxParent] isKindOfClass:[mozRootAccessible class]]) ||
+       [[self moxParent] isKindOfClass:[ChildView class]])) {
     // Our actual announcement should be stored as a child of the alert,
     // so we verify a child exists, and then query that child below.
     NSArray* children = [self moxChildren];
@@ -896,23 +899,24 @@ struct RoleDescrComparator {
     } else {
       key = [announcement moxTitle];
     }
-
     NSDictionary* info = @{
       NSAccessibilityAnnouncementKey : key ? key : @(""),
-      NSAccessibilityPriorityKey : @(NSAccessibilityPriorityMedium)
+      // High priority means VO will stop what it is currently speaking
+      // to speak our announcement.
+      NSAccessibilityPriorityKey : @(NSAccessibilityPriorityHigh)
     };
 
-    id window = [self moxWindow];
-
     // This sends events via nsIObserverService to be consumed by our
     // mochitests. Normally we'd fire these events through moxPostNotification
-    // which takes care of this, but because the window we fetch above isn't
+    // which takes care of this, but because NSApp isn't derived
-    // derrived from MOXAccessibleBase, we do this (and post the notification)
+    // from MOXAccessibleBase, we do this (and post the notification) manually.
-    // manually.
+    // We used to fire this on the window, but per Chrome and Safari these
+    // notifs get dropped if fired on any non-main window. We now fire on NSApp
+    // to avoid this.
     xpcAccessibleMacEvent::FireEvent(
-        window, NSAccessibilityAnnouncementRequestedNotification, info);
+        NSApp, NSAccessibilityAnnouncementRequestedNotification, info);
     NSAccessibilityPostNotificationWithUserInfo(
-        window, NSAccessibilityAnnouncementRequestedNotification, info);
+        NSApp, NSAccessibilityAnnouncementRequestedNotification, info);
   }
 }