Fix focus management for text fields (#51009)

Fix focus management for text fields

This PR:

1. Refactors the DOM `focus` function to take [options][1]
2. Removes the timers sorrounding the `activeDomElement.focus()` so that the input elements get focused as immediate as possible.
3. Prevents the default on pointerdown in a Flutter View and schedules a `requestViewFocusChange` to claim focus in that view after some time. This gives `2` the opportunity to focus the right `<input />` or `<textarea />` element. This helps focus correctly transition from one input element to another (without jumping to a flutter view in between).
4. Deactivating a `TextField` doesn't blur the focused element anymore, it insteads schedules for later a call to move the focus to the flutter view if nothing inside it claimed focus first.
5. Prevents scroll in all the focus calls (this should help with the view jumping when focusing one text field after another).

## Sample apps

1. Full screen mode: https://tugorez.com/flutter_focus_web
2. Embedded mode: https://tugorez.com/flutter_focus_web?embedded

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

Author: tugorez

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

@@ -16,7 +16,7 @@ final class ViewFocusBinding {
   ///
   /// DO NOT rely on this bit as it will go away soon. You're warned :)!
   @visibleForTesting
-  static bool isEnabled = false;
+  static bool isEnabled = true;
 
   final FlutterViewManager _viewManager;
   final ui.ViewFocusChangeCallback _onViewFocusChange;
@@ -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