Pass _caretPrototype to prevent cache miss (#46720)

dev/benchmarks/macrobenchmarks/lib/common.dart

@@ -9,3 +9,4 @@ const String kPostBackdropFilterRouteName = '/post_backdrop_filter';
 const String kSimpleAnimationRouteName = '/simple_animation';
 const String kPictureCacheRouteName = '/picture_cache';
 const String kLargeImagesRouteName = '/large_images';
+const String kTextRouteName = '/text';

dev/benchmarks/macrobenchmarks/lib/main.dart

@@ -12,6 +12,7 @@ import 'src/cubic_bezier.dart';
 import 'src/cull_opacity.dart';
 import 'src/post_backdrop_filter.dart';
 import 'src/simple_animation.dart';
+import 'src/text.dart';
 
 const String kMacrobenchmarks ='Macrobenchmarks';
 
@@ -34,6 +35,7 @@ class MacrobenchmarksApp extends StatelessWidget {
         kSimpleAnimationRouteName: (BuildContext conttext) => SimpleAnimationPage(),
         kPictureCacheRouteName: (BuildContext context) => PictureCachePage(),
         kLargeImagesRouteName: (BuildContext context) => LargeImagesPage(),
+        kTextRouteName: (BuildContext context) => TextPage(),
       },
     );
   }
@@ -97,6 +99,13 @@ class HomePage extends StatelessWidget {
               Navigator.pushNamed(context, kLargeImagesRouteName);
             },
           ),
+          RaisedButton(
+            key: const Key(kTextRouteName),
+            child: const Text('Text'),
+            onPressed: () {
+              Navigator.pushNamed(context, kTextRouteName);
+            },
+          ),
         ],
       ),
     );

dev/benchmarks/macrobenchmarks/lib/src/text.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+class TextPage extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: <Widget>[
+        Container(
+          width: 200,
+          height: 100,
+          child: const TextField(
+            key: Key('basic-textfield'),
+          ),
+        ),
+      ],
+    );
+  }
+}

dev/benchmarks/macrobenchmarks/test_driver/textfield_perf.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_driver/driver_extension.dart';
+import 'package:macrobenchmarks/main.dart' as app;
+
+void main() {
+  enableFlutterDriverExtension();
+  app.main();
+}

dev/benchmarks/macrobenchmarks/test_driver/textfield_perf_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_driver/flutter_driver.dart';
+import 'package:macrobenchmarks/common.dart';
+
+import 'util.dart';
+
+void main() {
+  macroPerfTest(
+    'textfield_perf',
+    kTextRouteName,
+    driverOps: (FlutterDriver driver) async {
+      final SerializableFinder textfield = find.byValueKey('basic-textfield');
+      driver.tap(textfield);
+      // Caret should be cached, so repeated blinking should not require recompute.
+      await Future<void>.delayed(const Duration(milliseconds: 5000));
+    },
+  );
+}

dev/devicelab/bin/tasks/textfield_perf.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter_devicelab/tasks/perf_tests.dart';
+import 'package:flutter_devicelab/framework/adb.dart';
+import 'package:flutter_devicelab/framework/framework.dart';
+
+Future<void> main() async {
+  deviceOperatingSystem = DeviceOperatingSystem.android;
+  await task(createTextfieldPerfTest());
+}

dev/devicelab/lib/tasks/perf_tests.dart

@@ -156,6 +156,14 @@ TaskFunction createBasicMaterialCompileTest() {
   };
 }
 
+TaskFunction createTextfieldPerfTest() {
+  return PerfTest(
+    '${flutterDirectory.path}/dev/benchmarks/macrobenchmarks',
+    'test_driver/textfield_perf.dart',
+    'textfield_perf',
+  ).run;
+}
+
 
 /// Measure application startup performance.
 class StartupTest {

packages/flutter/lib/src/painting/text_painter.dart

@@ -170,6 +170,8 @@ class TextPainter {
   void markNeedsLayout() {
     _paragraph = null;
     _needsLayout = true;
+    _previousCaretPosition = null;
+    _previousCaretPrototype = null;
   }
 
   /// The (potentially styled) text to paint.

packages/flutter/lib/src/rendering/editable.dart

@@ -365,7 +365,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
 
     final Offset startOffset = _textPainter.getOffsetForCaret(
       TextPosition(offset: _selection.start, affinity: _selection.affinity),
-      Rect.zero,
+      _caretPrototype,
     );
     // TODO(justinmc): https://github.com/flutter/flutter/issues/31495
     // Check if the selection is visible with an approximation because a
@@ -381,7 +381,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
 
     final Offset endOffset =  _textPainter.getOffsetForCaret(
       TextPosition(offset: _selection.end, affinity: _selection.affinity),
-      Rect.zero,
+      _caretPrototype,
     );
     _selectionEndInViewport.value = visibleRegion
       .inflate(visibleRegionSlop)