fix editable placement in a11y mode

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

@@ -4,6 +4,70 @@
 
 part of engine;
 
+/// Text editing used by accesibility mode.
+///
+/// [SemanticsTextEditingStrategy] assumes the caller will own the creation,
+/// insertion and disposal of the DOM element. Due to this
+/// [initializeElementPlacement], [initializeTextEditing] and
+/// [disable] strategies are handled differently.
+///
+/// This class is still responsible for hooking up the DOM element with the
+/// [HybridTextEditing] instance so that changes are communicated to Flutter.
+class SemanticsTextEditingStrategy extends DefaultTextEditingStrategy {
+  /// Creates a [SemanticsTextEditingStrategy] that eagerly instantiates
+  /// [domElement] so the caller can insert it before calling
+  /// [SemanticsTextEditingStrategy.enable].
+  SemanticsTextEditingStrategy(
+      HybridTextEditing owner, html.HtmlElement domElement)
+      : super(owner) {
+    // Make sure the DOM element is of a type that we support for text editing.
+    // TODO(yjbanov): move into initializer list when https://github.com/dart-lang/sdk/issues/37881 is fixed.
+    assert((domElement is html.InputElement) ||
+        (domElement is html.TextAreaElement));
+    super.domElement = domElement;
+  }
+
+  @override
+  void disable() {
+    // We don't want to remove the DOM element because the caller is responsible
+    // for that.
+    //
+    // Remove focus from the editable element to cause the keyboard to hide.
+    // Otherwise, the keyboard stays on screen even when the user navigates to
+    // a different screen (e.g. by hitting the "back" button).
+    domElement.blur();
+  }
+
+  @override
+  void initializeElementPlacement() {
+    // Element placement is done by [TextField].
+  }
+
+  @override
+  void initializeTextEditing(InputConfiguration inputConfig,
+      {_OnChangeCallback onChange, _OnActionCallback onAction}) {
+    // In accesibilty mode, the user of this class is supposed to insert the
+    // [domElement] on their own. Let's make sure they did.
+    assert(domElement != null);
+    assert(html.document.body.contains(domElement));
+
+    isEnabled = true;
+    _inputConfiguration = inputConfig;
+    _onChange = onChange;
+    _onAction = onAction;
+
+    domElement.focus();
+  }
+
+  @override
+  void setEditingState(EditingState editingState) {
+    super.setEditingState(editingState);
+
+    // Refocus after setting editing state.
+    domElement.focus();
+  }
+}
+
 /// Manages semantics objects that represent editable text fields.
 ///
 /// This role is implemented via a content-editable HTML element. This role does
@@ -19,15 +83,15 @@ class TextField extends RoleManager {
         semanticsObject.hasFlag(ui.SemanticsFlag.isMultiline)
             ? html.TextAreaElement()
             : html.InputElement();
-    persistentTextEditingElement = PersistentTextEditingElement(
+    textEditingElement = SemanticsTextEditingStrategy(
       textEditing,
       editableDomElement,
     );
     _setupDomElement();
   }
 
-  PersistentTextEditingElement persistentTextEditingElement;
-  html.Element get _textFieldElement => persistentTextEditingElement.domElement;
+  SemanticsTextEditingStrategy textEditingElement;
+  html.Element get _textFieldElement => textEditingElement.domElement;
 
   void _setupDomElement() {
     // On iOS, even though the semantic text field is transparent, the cursor
@@ -61,6 +125,7 @@ class TextField extends RoleManager {
     switch (browserEngine) {
       case BrowserEngine.blink:
       case BrowserEngine.edge:
+      case BrowserEngine.ie11:
       case BrowserEngine.firefox:
       case BrowserEngine.ie11:
       case BrowserEngine.unknown:
@@ -82,7 +147,7 @@ class TextField extends RoleManager {
         return;
       }
 
-      textEditing.useCustomEditableElement(persistentTextEditingElement);
+      textEditing.useCustomEditableElement(textEditingElement);
       ui.window
           .onSemanticsAction(semanticsObject.id, ui.SemanticsAction.tap, null);
     });
@@ -98,7 +163,7 @@ class TextField extends RoleManager {
     num lastTouchStartOffsetY;
 
     _textFieldElement.addEventListener('touchstart', (html.Event event) {
-      textEditing.useCustomEditableElement(persistentTextEditingElement);
+      textEditing.useCustomEditableElement(textEditingElement);
       final html.TouchEvent touchEvent = event;
       lastTouchStartOffsetX = touchEvent.changedTouches.last.client.x;
       lastTouchStartOffsetY = touchEvent.changedTouches.last.client.y;