AX: Sometimes web content becomes inaccessible to VoiceOver

https://bugs.webkit.org/show_bug.cgi?id=261006
rdar://113994918

Reviewed by Tyler Wilcock.

The problem is caused by the following sequence of events:
1. An AXObjectCache and AXIsolatedTree exist for the current page.
2. When the page is reloaded, the cache is destroyed and the isolated tree ActivityState is set to an empty set. The isolated tree is also queued to be destroyed.
3. VoiceOver queries for the focus off the main thread, but no isolated tree with focus is found sin the ActivityState is now empty or the tree was completely removed at that point.
4. A new AXObjectcache is created, but no new isolated object is created because this new AXObjectCache does not receive any request for the focus, which is one of the triggers of creating a new isolated tree.
The solution in this patch is to dispatch to the main thread the request for the focus in this scenario, which will result in the creation of a new isolated tree.

* Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm:
(WebKit::WebProcess::accessibilityFocusedUIElement):

Canonical link: https://commits.webkit.org/267562@main

Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm

@@ -209,6 +209,15 @@
 
 id WebProcess::accessibilityFocusedUIElement()
 {
+    auto retrieveFocusedUIElementFromMainThread = [] () {
+        return Accessibility::retrieveAutoreleasedValueFromMainThread<id>([] () -> RetainPtr<id> {
+            RefPtr page = WebProcess::singleton().focusedWebPage();
+            if (!page || !page->accessibilityRemoteObject())
+                return nil;
+            return [page->accessibilityRemoteObject() accessibilityFocusedUIElement];
+        });
+    };
+
 #if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
     if (!isMainRunLoop()) {
         // Avoid hitting the main thread by getting the focused object from the focused isolated tree.
@@ -223,23 +232,19 @@
             );
             return state.containsAll({ ActivityState::IsVisible, ActivityState::IsFocused, ActivityState::WindowIsActive });
         });
+        auto* isolatedTree = std::get_if<RefPtr<AXIsolatedTree>>(&tree);
+        if (!isolatedTree) {
+            // There is no isolated tree that has focus. This may be because none has been created yet, or because the one previously focused is being destroyed.
+            // In any case, get the focus from the main thread.
+            return retrieveFocusedUIElementFromMainThread();
+        }
 
-        RefPtr object = switchOn(tree,
-            [] (RefPtr<AXIsolatedTree>& typedTree) -> RefPtr<AXIsolatedObject> {
-                return typedTree ? typedTree->focusedNode() : nullptr;
-            }
-            , [] (auto&) -> RefPtr<AXIsolatedObject> {
-                return nullptr;
-            }
-        );
+        RefPtr object = (*isolatedTree)->focusedNode();
         return object ? object->wrapper() : nil;
     }
 #endif
 
-    RefPtr page = WebProcess::singleton().focusedWebPage();
-    if (!page || !page->accessibilityRemoteObject())
-        return nil;
-    return [page->accessibilityRemoteObject() accessibilityFocusedUIElement];
+    return retrieveFocusedUIElementFromMainThread();
 }
 
 #if USE(APPKIT)