Fixing a text editing bug happening when text field changes. (#41108)

* Carrying last size and transform information to TextInputConnection. Doing this we are making sure that the size/transform information from the previous connection will be removed, when connection changes.

* remove unused lastsize and lasttransform values

* Adding unit tests. Adressing comments.

packages/flutter/lib/src/services/text_input.dart

@@ -642,8 +642,11 @@ abstract class TextInputClient {
 ///  * [TextInput.attach]
 class TextInputConnection {
   TextInputConnection._(this._client)
-    : assert(_client != null),
-      _id = _nextId++;
+      : assert(_client != null),
+        _id = _nextId++;
+
+  Size _cachedSize;
+  Matrix4 _cachedTransform;
 
   static int _nextId = 1;
   final int _id;
@@ -678,14 +681,18 @@ class TextInputConnection {
   /// 2. [transform]: a matrix that maps the local paint coordinate system
   ///                 to the [PipelineOwner.rootNode].
   void setEditableSizeAndTransform(Size editableBoxSize, Matrix4 transform) {
-    SystemChannels.textInput.invokeMethod<void>(
-      'TextInput.setEditableSizeAndTransform',
-      <String, dynamic>{
-        'width': editableBoxSize.width,
-        'height': editableBoxSize.height,
-        'transform': transform.storage,
-      },
-    );
+    if (editableBoxSize != _cachedSize || transform != _cachedTransform) {
+      _cachedSize = editableBoxSize;
+      _cachedTransform = transform;
+      SystemChannels.textInput.invokeMethod<void>(
+        'TextInput.setEditableSizeAndTransform',
+        <String, dynamic>{
+          'width': editableBoxSize.width,
+          'height': editableBoxSize.height,
+          'transform': transform.storage,
+        },
+      );
+    }
   }
 
   /// Send text styling information.

packages/flutter/lib/src/widgets/editable_text.dart

@@ -1703,18 +1703,11 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     updateKeepAlive();
   }
 
-  Size _lastSize;
-  Matrix4 _lastTransform;
-
   void _updateSizeAndTransform() {
     if (_hasInputConnection) {
       final Size size = renderEditable.size;
       final Matrix4 transform = renderEditable.getTransformTo(null);
-      if (size != _lastSize || transform != _lastTransform) {
-        _lastSize = size;
-        _lastTransform = transform;
-        _textInputConnection.setEditableSizeAndTransform(size, transform);
-      }
+      _textInputConnection.setEditableSizeAndTransform(size, transform);
       SchedulerBinding.instance
           .addPostFrameCallback((Duration _) => _updateSizeAndTransform());
     }

packages/flutter/test/widgets/editable_text_test.dart

@@ -2215,6 +2215,88 @@ void main() {
     );
   });
 
+  testWidgets('transform and size is reset when text connection opens', (WidgetTester tester) async {
+    final List<MethodCall> log = <MethodCall>[];
+    SystemChannels.textInput.setMockMethodCallHandler((MethodCall methodCall) async {
+      log.add(methodCall);
+    });
+
+    final TextEditingController controller1 = TextEditingController();
+    final TextEditingController controller2 = TextEditingController();
+    controller1.text = 'Text1';
+    controller2.text = 'Text2';
+
+    await tester.pumpWidget(
+      MediaQuery(
+        data: const MediaQueryData(
+            devicePixelRatio: 1.0
+        ),
+        child: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.start,
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children:  <Widget>[
+              EditableText(
+                key: ValueKey<String>(controller1.text),
+                controller: controller1,
+                focusNode: FocusNode(),
+                style: Typography(platform: TargetPlatform.android).black.subhead,
+                cursorColor: Colors.blue,
+                backgroundCursorColor: Colors.grey,
+              ),
+              const SizedBox(height: 200.0),
+              EditableText(
+                key: ValueKey<String>(controller2.text),
+                controller: controller2,
+                focusNode: FocusNode(),
+                style: Typography(platform: TargetPlatform.android).black.subhead,
+                cursorColor: Colors.blue,
+                backgroundCursorColor: Colors.grey,
+              ),
+              const SizedBox(height: 100.0),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    await tester.showKeyboard(find.byKey(ValueKey<String>(controller1.text)));
+    final MethodCall methodCall = log.firstWhere((MethodCall m) => m.method == 'TextInput.setEditableSizeAndTransform');
+    expect(
+      methodCall,
+      isMethodCall('TextInput.setEditableSizeAndTransform', arguments: <String, dynamic>{
+        'width': 800,
+        'height': 14,
+        'transform': Matrix4.identity().storage.toList(),
+      }),
+    );
+
+    // Move to the next editable text.
+    await tester.showKeyboard(find.byKey(ValueKey<String>(controller2.text)));
+    final MethodCall methodCall2 = log.firstWhere((MethodCall m) => m.method == 'TextInput.setEditableSizeAndTransform');
+    expect(
+      methodCall2,
+      isMethodCall('TextInput.setEditableSizeAndTransform', arguments: <String, dynamic>{
+        'width': 800,
+        'height': 14,
+        'transform': Matrix4.identity().storage.toList(),
+      }),
+    );
+
+    // Move back to the first editable text.
+    await tester.showKeyboard(find.byKey(ValueKey<String>(controller1.text)));
+    final MethodCall methodCall3 = log.firstWhere((MethodCall m) => m.method == 'TextInput.setEditableSizeAndTransform');
+    expect(
+      methodCall3,
+      isMethodCall('TextInput.setEditableSizeAndTransform', arguments: <String, dynamic>{
+        'width': 800,
+        'height': 14,
+        'transform': Matrix4.identity().storage.toList(),
+      }),
+    );
+  });
+
   testWidgets('size and transform are sent when they change', (WidgetTester tester) async {
     final List<MethodCall> log = <MethodCall>[];
     SystemChannels.textInput.setMockMethodCallHandler((MethodCall methodCall) async {