[web] Reland "Fix focus management for text fields (#51009)" (#53537)

Relands: [**Fix focus management for text fields** (#51009)](#51009) by:

* Reverting commit: cf3ac2d (#53502).
* Keeping the new `ViewFocusBinding` disabled, as [suggested](#51009 (comment)).

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

lib/web_ui/lib/src/engine/dom.dart

@@ -658,7 +658,16 @@ extension DomElementExtension on DomElement {
   external JSNumber? get _tabIndex;
   double? get tabIndex => _tabIndex?.toDartDouble;
 
-  external JSVoid focus();
+  @JS('focus')
+  external JSVoid _focus(JSAny options);
+
+  void focus({bool? preventScroll, bool? focusVisible}) {
+    final Map<String, bool> options = <String, bool>{
+      if (preventScroll != null) 'preventScroll': preventScroll,
+      if (focusVisible != null) 'focusVisible': focusVisible,
+    };
+    _focus(options.toJSAnyDeep);
+  }
 
   @JS('scrollTop')
   external JSNumber get _scrollTop;
@@ -2249,9 +2258,11 @@ extension DomKeyboardEventExtension on DomKeyboardEvent {
   external JSBoolean? get _repeat;
   bool? get repeat => _repeat?.toDart;
 
+  // Safari injects synthetic keyboard events after auto-complete that don't
+  // have a `shiftKey` attribute, so this property must be nullable.
   @JS('shiftKey')
-  external JSBoolean get _shiftKey;
-  bool get shiftKey => _shiftKey.toDart;
+  external JSBoolean? get _shiftKey;
+  bool? get shiftKey => _shiftKey?.toDart;
 
   @JS('isComposing')
   external JSBoolean get _isComposing;

lib/web_ui/lib/src/engine/keyboard_binding.dart

@@ -207,7 +207,7 @@ class FlutterHtmlKeyboardEvent {
   num? get timeStamp => _event.timeStamp;
   bool get altKey => _event.altKey;
   bool get ctrlKey => _event.ctrlKey;
-  bool get shiftKey => _event.shiftKey;
+  bool get shiftKey => _event.shiftKey ?? false;
   bool get metaKey => _event.metaKey;
   bool get isComposing => _event.isComposing;
 

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

@@ -51,7 +51,7 @@ final class ViewFocusBinding {
     if (state == ui.ViewFocusState.focused) {
       // Only move the focus to the flutter view if nothing inside it is focused already.
       if (viewId != _viewId(domDocument.activeElement)) {
-        viewElement?.focus();
+        viewElement?.focus(preventScroll: true);
       }
     } else {
       viewElement?.blur();
@@ -70,7 +70,7 @@ final class ViewFocusBinding {
 
   late final DomEventListener _handleKeyDown = createDomEventListener((DomEvent event) {
     event as DomKeyboardEvent;
-    if (event.shiftKey) {
+    if (event.shiftKey ?? false) {
       _viewFocusDirection = ui.ViewFocusDirection.backward;
     }
   });

lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -982,6 +982,22 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
         );
       _convertEventsToPointerData(data: pointerData, event: event, details: down);
       _callback(event, pointerData);
+
+      if (event.target == _viewTarget) {
+        // Ensure smooth focus transitions between text fields within the Flutter view.
+        // Without preventing the default and this delay, the engine may not have fully
+        // rendered the next input element, leading to the focus incorrectly returning to
+        // the main Flutter view instead.
+        // A zero-length timer is sufficient in all tested browsers to achieve this.
+        event.preventDefault();
+        Timer(Duration.zero, () {
+          EnginePlatformDispatcher.instance.requestViewFocusChange(
+            viewId: _view.viewId,
+            state: ui.ViewFocusState.focused,
+            direction: ui.ViewFocusDirection.undefined,
+          );
+        });
+      }
     });
 
     // Why `domWindow` you ask? See this fiddle: https://jsfiddle.net/ditman/7towxaqp