[web_ui] Fixing invalid state bug for text editing (#12698)

* Fixing invalid state bug for text editing. Flutter Framework was sending editing state selection base and extent as -1. Since -1 is an invalid value for a dom element selection it was not applied to the last editing state. Now if the base or offset is sent as negative value, web engine will set 0 to the selection range.

* Addressing PR comments.

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

@@ -61,10 +61,20 @@ class EditingState {
   ///   "composingExtent": -1
   /// }
   /// ```
-  EditingState.fromFlutter(Map<String, dynamic> flutterEditingState)
-      : text = flutterEditingState['text'],
-        baseOffset = flutterEditingState['selectionBase'],
-        extentOffset = flutterEditingState['selectionExtent'];
+  ///
+  /// Flutter Framework can send the [selectionBase] and [selectionExtent] as
+  /// -1, if so 0 assigned to the [baseOffset] and [extentOffset]. -1 is not a
+  /// valid selection range for input DOM elements.
+  factory EditingState.fromFlutter(Map<String, dynamic> flutterEditingState) {
+    final int selectionBase = flutterEditingState['selectionBase'];
+    final int selectionExtent = flutterEditingState['selectionExtent'];
+    final String text = flutterEditingState['text'];
+
+    return EditingState(
+        text: text,
+        baseOffset: math.max(0, selectionBase),
+        extentOffset: math.max(0, selectionExtent));
+  }
 
   /// Creates an [EditingState] instance using values from the editing element
   /// in the DOM.

lib/web_ui/test/text_editing_test.dart

@@ -549,6 +549,47 @@ void main() {
       expect(spy.messages, isEmpty);
     });
 
+    test(
+        'negative base offset and selection extent values in editing state is handled',
+        () {
+      final MethodCall setClient = MethodCall(
+          'TextInput.setClient', <dynamic>[123, flutterSinglelineConfig]);
+      textEditing.handleTextInput(codec.encodeMethodCall(setClient));
+
+      const MethodCall setEditingState1 =
+          MethodCall('TextInput.setEditingState', <String, dynamic>{
+        'text': 'xyz',
+        'selectionBase': 1,
+        'selectionExtent': 2,
+      });
+      textEditing.handleTextInput(codec.encodeMethodCall(setEditingState1));
+
+      const MethodCall show = MethodCall('TextInput.show');
+      textEditing.handleTextInput(codec.encodeMethodCall(show));
+
+      // Check if the selection range is correct.
+      checkInputEditingState(
+          textEditing.editingElement.domElement, 'xyz', 1, 2);
+
+      const MethodCall setEditingState2 =
+          MethodCall('TextInput.setEditingState', <String, dynamic>{
+        'text': 'xyz',
+        'selectionBase': -1,
+        'selectionExtent': -1,
+      });
+      textEditing.handleTextInput(codec.encodeMethodCall(setEditingState2));
+
+      // The negative offset values are applied to the dom element as 0.
+      checkInputEditingState(
+          textEditing.editingElement.domElement, 'xyz', 0, 0);
+
+      const MethodCall clearClient = MethodCall('TextInput.clearClient');
+      textEditing.handleTextInput(codec.encodeMethodCall(clearClient));
+
+      // Confirm that [HybridTextEditing] didn't send any messages.
+      expect(spy.messages, isEmpty);
+    });
+
     test('Syncs the editing state back to Flutter', () {
       final MethodCall setClient = MethodCall(
           'TextInput.setClient', <dynamic>[123, flutterSinglelineConfig]);