Reland: wait until safe time to create root accessibility object

It is only safe to create an accessibility object when a document's
style has been computed and layout is clean.

Previously, tests failed because an extra kLoadComplete event
was sometimes fired for the initial empty document, before navigating
to the actual data: url with the test's contents. The extra event
only seems to occur in ChromeOS debug builds.

Bug: 1156939
Change-Id: I95f4afdb5471a98ed87fb1cf8e8a208e5ebfc453
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611559Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842329}

content/renderer/accessibility/render_accessibility_impl.cc

@@ -265,14 +265,9 @@ void RenderAccessibilityImpl::AccessibilityModeChanged(const ui::AXMode& mode) {
   if (!document.IsNull()) {
     StartOrStopLabelingImages(old_mode, mode);
 
-    // If there are any events in flight, |HandleAXEvent| will refuse to process
-    // our new event.
-    pending_events_.clear();
-    auto root_object = WebAXObject::FromWebDocument(document, false);
-    ax::mojom::Event event = root_object.IsLoaded()
-                                 ? ax::mojom::Event::kLoadComplete
-                                 : ax::mojom::Event::kLayoutComplete;
-    HandleAXEvent(ui::AXEvent(root_object.AxID(), event));
+    needs_initial_ax_tree_root_ = true;
+    event_schedule_mode_ = EventScheduleMode::kProcessEventsImmediately;
+    ScheduleSendPendingAccessibilityEvents();
   }
 }
 
@@ -476,11 +471,11 @@ void RenderAccessibilityImpl::Reset(int32_t reset_token) {
   if (!document.IsNull()) {
     // Tree-only mode gets used by the automation extension API which requires a
     // load complete event to invoke listener callbacks.
-    auto root_object = WebAXObject::FromWebDocument(document, false);
-    ax::mojom::Event event = root_object.IsLoaded()
-                                 ? ax::mojom::Event::kLoadComplete
-                                 : ax::mojom::Event::kLayoutComplete;
-    HandleAXEvent(ui::AXEvent(root_object.AxID(), event));
+    // SendPendingAccessibilityEvents() will fire the load complete event
+    // if the page is loaded.
+    needs_initial_ax_tree_root_ = true;
+    event_schedule_mode_ = EventScheduleMode::kProcessEventsImmediately;
+    ScheduleSendPendingAccessibilityEvents();
   }
 }
 
@@ -797,10 +792,20 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
     // complete for the entire document, in order to initialize the browser's
     // cached accessibility tree.
     needs_initial_ax_tree_root_ = false;
-    auto obj = WebAXObject::FromWebDocument(document);
+    auto root_obj = WebAXObject::FromWebDocument(document);
+    // Always fire layout complete for a new root object.
     pending_events_.insert(
         pending_events_.begin(),
-        ui::AXEvent(obj.AxID(), ax::mojom::Event::kLayoutComplete));
+        ui::AXEvent(root_obj.AxID(), ax::mojom::Event::kLayoutComplete));
+
+    // If loaded and has some content, insert load complete at the top, so that
+    // screen readers are informed a new document is ready.
+    if (root_obj.IsLoaded() && !document.Body().IsNull() &&
+        !document.Body().FirstChild().IsNull()) {
+      pending_events_.insert(
+          pending_events_.begin(),
+          ui::AXEvent(root_obj.AxID(), ax::mojom::Event::kLoadComplete));
+    }
   }
 
   if (pending_events_.empty() && dirty_objects_.empty()) {
@@ -834,8 +839,8 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
   // location changes.
   bool need_to_send_location_changes = false;
 
-  // If there's a load complete message, we need to change the event schedule
-  // mode.
+  // Keep track of load complete messages. When a load completes, it's a good
+  // time to inject a stylesheet for image annotation debugging.
   bool had_load_complete_messages = false;
 
   ScopedFreezeBlinkAXTreeSource freeze(tree_source_.get());

third_party/blink/public/web/web_ax_object.h

@@ -81,9 +81,7 @@ class WebAXObject {
   BLINK_EXPORT bool operator>(const WebAXObject& other) const;
   BLINK_EXPORT bool operator>=(const WebAXObject& other) const;
   BLINK_EXPORT static WebAXObject FromWebNode(const WebNode&);
-  BLINK_EXPORT static WebAXObject FromWebDocument(
-      const WebDocument&,
-      bool update_layout_if_necessary = true);
+  BLINK_EXPORT static WebAXObject FromWebDocument(const WebDocument&);
   BLINK_EXPORT static WebAXObject FromWebDocumentByID(const WebDocument&, int);
   BLINK_EXPORT static WebAXObject FromWebDocumentFocused(
       const WebDocument&,

third_party/blink/renderer/modules/exported/web_ax_object.cc

@@ -1443,12 +1443,9 @@ WebAXObject WebAXObject::FromWebNode(const WebNode& web_node) {
 }
 
 // static
-WebAXObject WebAXObject::FromWebDocument(const WebDocument& web_document,
-                                         bool update_layout_if_necessary) {
-  if (update_layout_if_necessary &&
-      !MaybeUpdateLayoutAndCheckValidity(web_document)) {
+WebAXObject WebAXObject::FromWebDocument(const WebDocument& web_document) {
+  if (!MaybeUpdateLayoutAndCheckValidity(web_document))
     return WebAXObject();
-  }
   const Document* document = web_document.ConstUnwrap<Document>();
   auto* cache = To<AXObjectCacheImpl>(document->ExistingAXObjectCache());
   return cache ? WebAXObject(cache->GetOrCreate(document->GetLayoutView()))