[web] reland fix text selection offset in multi-line fields (#166714)

Fixes https://github.com/flutter/flutter/issues/162698

This reland contains the original changes from
https://github.com/flutter/flutter/pull/166565, plus Safari-specific
test fixes.

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

@@ -543,6 +543,7 @@ extension type DomCSSStyleDeclaration._(JSObject _) implements JSObject {
   set textAlign(String value) => setProperty('text-align', value);
   set font(String value) => setProperty('font', value);
   set cursor(String value) => setProperty('cursor', value);
+  set scrollbarWidth(String value) => setProperty('scrollbar-width', value);
   String get width => getPropertyValue('width');
   String get height => getPropertyValue('height');
   String get position => getPropertyValue('position');
@@ -604,6 +605,7 @@ extension type DomCSSStyleDeclaration._(JSObject _) implements JSObject {
   String get textAlign => getPropertyValue('text-align');
   String get font => getPropertyValue('font');
   String get cursor => getPropertyValue('cursor');
+  String get scrollbarWidth => getPropertyValue('scrollbar-width');
 
   external String getPropertyValue(String property);
 

engine/src/flutter/lib/web_ui/lib/src/engine/semantics/text_field.dart

@@ -6,6 +6,7 @@ import 'package:ui/ui.dart' as ui;
 
 import '../dom.dart';
 import '../platform_dispatcher.dart';
+import '../text_editing/input_type.dart';
 import '../text_editing/text_editing.dart';
 import 'semantics.dart';
 
@@ -229,7 +230,7 @@ class SemanticTextField extends SemanticRole {
   }
 
   DomHTMLTextAreaElement _createMultiLineField() {
-    final textArea = createDomHTMLTextAreaElement();
+    final textArea = createMultilineTextArea();
 
     if (semanticsObject.hasFlag(ui.SemanticsFlag.isObscured)) {
       // -webkit-text-security is not standard, but it's the best we can do.

engine/src/flutter/lib/web_ui/lib/src/engine/text_editing/input_type.dart

@@ -120,7 +120,7 @@ class MultilineNoTextInputType extends MultilineInputType {
   String? get inputmodeAttribute => 'none';
 
   @override
-  DomHTMLElement createDomElement() => createDomHTMLTextAreaElement();
+  DomHTMLElement createDomElement() => createMultilineTextArea();
 }
 
 /// Single-line text input type.
@@ -184,5 +184,12 @@ class MultilineInputType extends EngineInputType {
   String? get inputmodeAttribute => null;
 
   @override
-  DomHTMLElement createDomElement() => createDomHTMLTextAreaElement();
+  DomHTMLElement createDomElement() => createMultilineTextArea();
+}
+
+DomHTMLTextAreaElement createMultilineTextArea() {
+  final element = createDomHTMLTextAreaElement();
+  // Scrollbar width affects text layout. This zeroes out the scrollbar width.
+  element.style.scrollbarWidth = 'none';
+  return element;
 }

engine/src/flutter/lib/web_ui/lib/src/engine/text_editing/text_editing.dart

@@ -64,7 +64,6 @@ void _setStaticStyleAttributes(DomHTMLElement domElement) {
     // For more details, see: https://developer.mozilla.org/en-US/docs/Web/CSS/forced-color-adjust
     ..setProperty('forced-color-adjust', 'none')
     ..whiteSpace = 'pre-wrap'
-    ..alignContent = 'center'
     ..position = 'absolute'
     ..top = '0'
     ..left = '0'
@@ -108,7 +107,6 @@ void _styleAutofillElements(
   final DomCSSStyleDeclaration elementStyle = domElement.style;
   elementStyle
     ..whiteSpace = 'pre-wrap'
-    ..alignContent = 'center'
     ..padding = '0'
     ..opacity = '1'
     ..color = 'transparent'

engine/src/flutter/lib/web_ui/test/engine/text_editing_test.dart

@@ -840,6 +840,56 @@ Future<void> testMain() async {
       expect(spy.messages, isEmpty);
     });
 
+    test('Does not align content in autofill group elements', () {
+      final setClient = MethodCall('TextInput.setClient', <dynamic>[
+        123,
+        createFlutterConfig('text'),
+      ]);
+      sendFrameworkMessage(codec.encodeMethodCall(setClient));
+
+      const setEditingState = MethodCall('TextInput.setEditingState', <String, dynamic>{
+        'text': 'abcd',
+        'selectionBase': 2,
+        'selectionExtent': 3,
+      });
+      sendFrameworkMessage(codec.encodeMethodCall(setEditingState));
+
+      const show = MethodCall('TextInput.show');
+      sendFrameworkMessage(codec.encodeMethodCall(show));
+
+      // The "setSizeAndTransform" message has to be here before we call
+      // checkInputEditingState, since on some platforms (e.g. Desktop Safari)
+      // we don't put the input element into the DOM until we get its correct
+      // dimensions from the framework.
+      final MethodCall setSizeAndTransform = configureSetSizeAndTransformMethodCall(
+        150,
+        50,
+        Matrix4.translationValues(10.0, 20.0, 30.0).storage.toList(),
+      );
+      sendFrameworkMessage(codec.encodeMethodCall(setSizeAndTransform));
+
+      // Form elements
+      {
+        final formElement = textEditing!.configuration!.autofillGroup!.formElement;
+        expect(formElement.style.alignContent, isEmpty);
+
+        // Should contain one <input type="text"> and one <input type="submit">
+        expect(formElement.children, hasLength(2));
+
+        final inputElement = formElement.children.first;
+        expect(inputElement.style.alignContent, isEmpty);
+
+        final submitElement = formElement.children.last;
+        expect(submitElement.style.alignContent, isEmpty);
+      }
+
+      // Active element
+      {
+        final DomHTMLElement activeElement = textEditing!.strategy.activeDomElement;
+        expect(activeElement.style.alignContent, isEmpty);
+      }
+    });
+
     test('focus and connection with blur', () async {
       // In all the desktop browsers we are keeping the connection
       // open, keep the text editing element focused if it receives a blur
@@ -3585,6 +3635,16 @@ Future<void> testMain() async {
       // though it supports forced-colors. Safari doesn't support forced-colors
       // so this isn't a problem there.
     }, skip: isFirefox || isSafari);
+
+    test('Multi-line text area scrollbars are zero-width', () {
+      final allowedScrollbarWidthValues = <String>[
+        'none',
+        // Safari introduced scrollbarWidth support in 18.2. Older Safari versions
+        // return empty string instead of 'none'.
+        if (isSafari) '',
+      ];
+      expect(allowedScrollbarWidthValues, contains(createMultilineTextArea().style.scrollbarWidth));
+    });
   });
 }