Call IAccessible::accFocus to move a11y focus (flutter#30256)

On receipt of a FOCUS_CHANGED event from the AX tree, call
IAccessible::accFocus to tell screen readers to move the accessibility
focus to that node. This is in addition to setting the keyboard focus
via the EVENT_OBJECT_FOCUS event.

Issue: flutter#77838

Author: cbracken

shell/platform/windows/accessibility_bridge_delegate_win32.cc

@@ -39,6 +39,7 @@ void AccessibilityBridgeDelegateWin32::OnAccessibilityEvent(
       break;
     case ui::AXEventGenerator::Event::FOCUS_CHANGED:
       DispatchWinAccessibilityEvent(win_delegate, EVENT_OBJECT_FOCUS);
+      SetFocus(win_delegate);
       break;
     case ui::AXEventGenerator::Event::IGNORED_CHANGED:
       if (ax_node->IsIgnored()) {
@@ -152,4 +153,9 @@ void AccessibilityBridgeDelegateWin32::DispatchWinAccessibilityEvent(
   node_delegate->DispatchWinAccessibilityEvent(event_type);
 }
 
+void AccessibilityBridgeDelegateWin32::SetFocus(
+    std::shared_ptr<FlutterPlatformNodeDelegateWin32> node_delegate) {
+  node_delegate->SetFocus();
+}
+
 }  // namespace flutter

shell/platform/windows/accessibility_bridge_delegate_win32.h

@@ -46,6 +46,11 @@ class AccessibilityBridgeDelegateWin32
       std::shared_ptr<FlutterPlatformNodeDelegateWin32> node_delegate,
       DWORD event_type);
 
+  // Sets the accessibility focus to the accessibility node associated with the
+  // specified semantics node.
+  virtual void SetFocus(
+      std::shared_ptr<FlutterPlatformNodeDelegateWin32> node_delegate);
+
  private:
   FlutterWindowsEngine* engine_;
 };

shell/platform/windows/accessibility_bridge_delegate_win32_unittests.cc

@@ -45,13 +45,25 @@ class AccessibilityBridgeDelegateWin32Spy
     dispatched_events_.push_back({node_delegate, event_type});
   }
 
-  void Reset() { dispatched_events_.clear(); }
+  void SetFocus(std::shared_ptr<FlutterPlatformNodeDelegateWin32> node_delegate)
+      override {
+    focused_nodes_.push_back(node_delegate->GetAXNode()->id());
+  }
+
+  void Reset() {
+    dispatched_events_.clear();
+    focused_nodes_.clear();
+  }
+
   const std::vector<MsaaEvent>& dispatched_events() const {
     return dispatched_events_;
-  };
+  }
+
+  const std::vector<int32_t> focused_nodes() const { return focused_nodes_; }
 
  private:
   std::vector<MsaaEvent> dispatched_events_;
+  std::vector<int32_t> focused_nodes_;
 };
 
 // Returns an engine instance configured with dummy project path values, and
@@ -203,8 +215,25 @@ TEST(AccessibilityBridgeDelegateWin32, OnAccessibilityEventChildrenChanged) {
 }
 
 TEST(AccessibilityBridgeDelegateWin32, OnAccessibilityEventFocusChanged) {
-  ExpectWinEventFromAXEvent(1, ui::AXEventGenerator::Event::FOCUS_CHANGED,
-                            EVENT_OBJECT_FOCUS);
+  auto window_binding_handler =
+      std::make_unique<::testing::NiceMock<MockWindowBindingHandler>>();
+  FlutterWindowsView view(std::move(window_binding_handler));
+  view.SetEngine(GetTestEngine());
+  view.OnUpdateSemanticsEnabled(true);
+
+  auto bridge = view.GetEngine()->accessibility_bridge().lock();
+  PopulateAXTree(bridge);
+
+  AccessibilityBridgeDelegateWin32Spy spy(view.GetEngine());
+  spy.OnAccessibilityEvent({AXNodeFromID(bridge, 1),
+                            {ui::AXEventGenerator::Event::FOCUS_CHANGED,
+                             ax::mojom::EventFrom::kNone,
+                             {}}});
+  ASSERT_EQ(spy.dispatched_events().size(), 1);
+  EXPECT_EQ(spy.dispatched_events()[0].event_type, EVENT_OBJECT_FOCUS);
+
+  ASSERT_EQ(spy.focused_nodes().size(), 1);
+  EXPECT_EQ(spy.focused_nodes()[0], 1);
 }
 
 TEST(AccessibilityBridgeDelegateWin32, OnAccessibilityEventIgnoredChanged) {

shell/platform/windows/flutter_platform_node_delegate_win32.cc

@@ -92,4 +92,11 @@ void FlutterPlatformNodeDelegateWin32::DispatchWinAccessibilityEvent(
                    -ax_platform_node_->GetUniqueId());
 }
 
+void FlutterPlatformNodeDelegateWin32::SetFocus() {
+  VARIANT varchild{};
+  varchild.vt = VT_I4;
+  varchild.lVal = CHILDID_SELF;
+  GetNativeViewAccessible()->accSelect(SELFLAG_TAKEFOCUS, varchild);
+}
+
 }  // namespace flutter

shell/platform/windows/flutter_platform_node_delegate_win32.h

@@ -45,6 +45,10 @@ class FlutterPlatformNodeDelegateWin32 : public FlutterPlatformNodeDelegate {
   // convenience wrapper around |NotifyWinEvent|.
   virtual void DispatchWinAccessibilityEvent(DWORD event_type);
 
+  // Sets the accessibility focus to the accessibility node associated with
+  // this object.
+  void SetFocus();
+
  private:
   ui::AXPlatformNode* ax_platform_node_;
   FlutterWindowsEngine* engine_;