Flutter views can gain focus (flutter/engine#54985)

I am unsure why the `tabindex` was removed when semantics were enabled. It seems this change was made without a clear explanation (by me). This PR shouldn't cause any issues as Flutter Views already have a tabindex, we're not adding a new one. This change is necessary because the semantics text strategy refocuses the view on deactivation, requiring the Flutter view to be focusable.

ThIs PR is a requirement to enable https://github.com/flutter/engine/pull/54966. 

https://github.com/flutter/flutter/issues/153022

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher/view_focus_binding.dart

@@ -27,18 +27,20 @@ final class ViewFocusBinding {
   StreamSubscription<int>? _onViewCreatedListener;
 
   void init() {
+    // We need a global listener here to know if the user was pressing "shift"
+    // when the Flutter view receives focus, to move the Flutter focus to the
+    // *last* focusable element.
     domDocument.body?.addEventListener(_keyDown, _handleKeyDown);
     domDocument.body?.addEventListener(_keyUp, _handleKeyUp);
-    domDocument.body?.addEventListener(_focusin, _handleFocusin);
+
-    domDocument.body?.addEventListener(_focusout, _handleFocusout);
+    // If so, update `_handleViewCreated` and add a `_handleViewDisposed` to attach
+    // and remove the focus/blur listener.
     _onViewCreatedListener = _viewManager.onViewCreated.listen(_handleViewCreated);
   }
 
   void dispose() {
     domDocument.body?.removeEventListener(_keyDown, _handleKeyDown);
     domDocument.body?.removeEventListener(_keyUp, _handleKeyUp);
-    domDocument.body?.removeEventListener(_focusin, _handleFocusin);
-    domDocument.body?.removeEventListener(_focusout, _handleFocusout);
     _onViewCreatedListener?.cancel();
   }
 
@@ -48,12 +50,13 @@ final class ViewFocusBinding {
     }
     final DomElement? viewElement = _viewManager[viewId]?.dom.rootElement;
 
-    if (state == ui.ViewFocusState.focused) {
+    switch (state) {
+      case ui.ViewFocusState.focused:
         // Only move the focus to the flutter view if nothing inside it is focused already.
         if (viewId != _viewId(domDocument.activeElement)) {
           viewElement?.focusWithoutScroll();
         }
-    } else {
+      case ui.ViewFocusState.unfocused:
         viewElement?.blur();
     }
   }
@@ -115,8 +118,8 @@ final class ViewFocusBinding {
         direction: _viewFocusDirection,
       );
     }
-    _maybeMarkViewAsFocusable(_lastViewId, reachableByKeyboard: true);
+    _updateViewKeyboardReachability(_lastViewId, reachable: true);
-    _maybeMarkViewAsFocusable(viewId, reachableByKeyboard: false);
+    _updateViewKeyboardReachability(viewId, reachable: false);
     _lastViewId = viewId;
     _onViewFocusChange(event);
   }
@@ -127,29 +130,32 @@ final class ViewFocusBinding {
   }
 
   void _handleViewCreated(int viewId) {
-    _maybeMarkViewAsFocusable(viewId, reachableByKeyboard: true);
+    final DomElement? rootElement = _viewManager[viewId]?.dom.rootElement;
+
+    rootElement?.addEventListener(_focusin, _handleFocusin);
+    rootElement?.addEventListener(_focusout, _handleFocusout);
+
+    _updateViewKeyboardReachability(viewId, reachable: true);
   }
 
-  void _maybeMarkViewAsFocusable(
+  // Controls whether the Flutter view identified by [viewId] is reachable by
+  // keyboard.
+  void _updateViewKeyboardReachability(
     int? viewId, {
-    required bool reachableByKeyboard,
+    required bool reachable,
   }) {
     if (viewId == null) {
       return;
     }
 
     final DomElement? rootElement = _viewManager[viewId]?.dom.rootElement;
-    if (EngineSemantics.instance.semanticsEnabled) {
+    // A tabindex with value zero means the DOM element can be reached using the
-      rootElement?.removeAttribute('tabindex');
+    // keyboard (tab, shift + tab). When its value is -1 it is still focusable
-    } else {
+    // but can't be focused as the result of keyboard events. This is specially
-      // A tabindex with value zero means the DOM element can be reached by using
-      // the keyboard (tab, shift + tab). When its value is -1 it is still focusable
-      // but can't be focused by the result of keyboard events This is specially
     // important when the semantics tree is enabled as it puts DOM nodes inside
     // the flutter view and having it with a zero tabindex messes the focus
     // traversal order when pressing tab or shift tab.
-      rootElement?.setAttribute('tabindex', reachableByKeyboard ? 0 : -1);
+    rootElement?.setAttribute('tabindex', reachable ? 0 : -1);
-    }
   }
 
   static const String _focusin = 'focusin';

engine/src/flutter/lib/web_ui/test/engine/platform_dispatcher/view_focus_binding_test.dart

@@ -68,28 +68,6 @@ void testMain() {
       expect(view2.dom.rootElement.getAttribute('tabindex'), '0');
     });
 
-    test('never marks the views as focusable with semantincs enabled', () async {
-      EngineSemantics.instance.semanticsEnabled = true;
-
-      final EngineFlutterView view1 = createAndRegisterView(dispatcher);
-      final EngineFlutterView view2 = createAndRegisterView(dispatcher);
-
-      expect(view1.dom.rootElement.getAttribute('tabindex'), isNull);
-      expect(view2.dom.rootElement.getAttribute('tabindex'), isNull);
-
-      view1.dom.rootElement.focusWithoutScroll();
-      expect(view1.dom.rootElement.getAttribute('tabindex'), isNull);
-      expect(view2.dom.rootElement.getAttribute('tabindex'), isNull);
-
-      view2.dom.rootElement.focusWithoutScroll();
-      expect(view1.dom.rootElement.getAttribute('tabindex'), isNull);
-      expect(view2.dom.rootElement.getAttribute('tabindex'), isNull);
-
-      view2.dom.rootElement.blur();
-      expect(view1.dom.rootElement.getAttribute('tabindex'), isNull);
-      expect(view2.dom.rootElement.getAttribute('tabindex'), isNull);
-    });
-
     test('fires a focus event - a view was focused', () async {
       final EngineFlutterView view = createAndRegisterView(dispatcher);