Add framework support for system text scale factor. (#12180)

* Add framework-side support for system text scale factor.

* Rolling engine to e3404b81a53ba3180c7623a6f2190ebb28518f30
  Additional changes rolled in with engine change:
    libtxt: implementation of GetRectsForRange that processes a line at a time - e3404b8
    Provide an entropy source to the Dart engine (#4161) - e1aa867
    libtxt: search for fallback fonts that can match emoji and CJK characters - 8061df1
    Roll skia to e4679fa06a. (#4157) - 267e7a8
    Update buildroot to 53fea9aebbcc39c6522731471a1a45960ee0685e (#4160) - 02ea7ae
    Revert engine Dart roll. (#4158) - 14aab33
    Add support for system text scale factor. (#4124) - b2a7f4b
    Include _http into sky_engine libraries for analyzer (#4154) - b930f10
    libtxt: Remove postprocess_line and improve tracking of X offsets - 86f95f0
    libtxt: remove redundant line_widths (#4152) - 14bf515
    Roll dart to ade37f931e90b0fdb8fe16d6bf6f089545da55b6 (#4151) - 6f1264f

bin/internal/engine.version

-bdfedcc26f78ea332cdac0ed67590157c82c954e
+e3404b81a53ba3180c7623a6f2190ebb28518f30
\ No newline at end of file

dev/benchmarks/complex_layout/android/app/src/main/AndroidManifest.xml

@@ -7,7 +7,7 @@
         <activity android:name="io.flutter.app.FlutterActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/benchmarks/microbenchmarks/android/app/src/main/AndroidManifest.xml

@@ -7,7 +7,7 @@
         <activity android:name="io.flutter.app.FlutterActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/integration_tests/channels/android/app/src/main/AndroidManifest.xml

@@ -16,7 +16,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/integration_tests/flavors/android/app/src/main/AndroidManifest.xml

@@ -9,7 +9,7 @@
         <activity
             android:name=".MainActivity"
             android:launchMode="singleTop"
-            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/integration_tests/platform_interaction/android/app/src/main/AndroidManifest.xml

@@ -16,7 +16,7 @@
         <activity android:name="com.yourcompany.platforminteraction.MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/integration_tests/ui/android/app/src/main/AndroidManifest.xml

@@ -20,7 +20,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

dev/manual_tests/android/app/src/main/AndroidManifest.xml

@@ -21,7 +21,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/catalog/android/app/src/main/AndroidManifest.xml

@@ -16,7 +16,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/flutter_gallery/android/app/src/main/AndroidManifest.xml

@@ -21,7 +21,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Light.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/flutter_view/android/app/src/main/AndroidManifest.xml

@@ -15,7 +15,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@style/Theme.AppCompat"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/hello_world/android/app/src/main/AndroidManifest.xml

@@ -21,7 +21,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/layers/android/app/src/main/AndroidManifest.xml

@@ -16,7 +16,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/platform_channel/android/app/src/main/AndroidManifest.xml

@@ -13,7 +13,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

examples/platform_view/android/app/src/main/AndroidManifest.xml

@@ -20,7 +20,7 @@
             android:name="io.flutter.examples.platform_view.MainActivity"
             android:launchMode="singleTop"
             android:theme="@android:style/Theme.Black.NoTitleBar"
-            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
             <intent-filter>
@@ -32,6 +32,6 @@
             android:name="io.flutter.examples.platform_view.CountActivity"
             android:parentActivityName="io.flutter.examples.platform_view.MainActivity"
             android:theme="@style/Theme.AppCompat.NoActionBar"
-            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"></activity>
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"></activity>
     </application>
 </manifest>

examples/stocks/android/app/src/main/AndroidManifest.xml

@@ -21,7 +21,7 @@
         <activity android:name=".MainActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
-                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+                  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
                   android:hardwareAccelerated="true"
                   android:windowSoftInputMode="adjustResize">
             <intent-filter>

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

@@ -245,8 +245,14 @@ class TextStyle extends Diagnosticable {
   /// During painting, the [fontSize] is multiplied by the current
   /// `textScaleFactor` to let users make it easier to read text by increasing
   /// its size.
+  ///
+  /// [getParagraphStyle] will default to 14 logical pixels if the font size
+  /// isn't specified here.
   final double fontSize;
 
+  // The default font size if none is specified.
+  static const double _defaultFontSize = 14.0;
+
   /// The typeface thickness to use when painting the text (e.g., bold).
   final FontWeight fontWeight;
 
@@ -444,6 +450,9 @@ class TextStyle extends Diagnosticable {
   /// The `textScaleFactor` argument must not be null. If omitted, it defaults
   /// to 1.0. The other arguments may be null. The `maxLines` argument, if
   /// specified and non-null, must be greater than zero.
+  ///
+  /// If the font size on this style isn't set, it will default to 14 logical
+  /// pixels.
   ui.ParagraphStyle getParagraphStyle({
       TextAlign textAlign,
       TextDirection textDirection,
@@ -459,7 +468,7 @@ class TextStyle extends Diagnosticable {
       fontWeight: fontWeight,
       fontStyle: fontStyle,
       fontFamily: fontFamily,
-      fontSize: fontSize == null ? null : fontSize * textScaleFactor,
+      fontSize: (fontSize ?? _defaultFontSize) * textScaleFactor,
       lineHeight: height,
       maxLines: maxLines,
       ellipsis: ellipsis,

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

@@ -36,6 +36,7 @@ abstract class RendererBinding extends BindingBase with SchedulerBinding, Servic
     );
     ui.window
       ..onMetricsChanged = handleMetricsChanged
+      ..onTextScaleFactorChanged = handleTextScaleFactorChanged
       ..onSemanticsEnabledChanged = _handleSemanticsEnabledChanged
       ..onSemanticsAction = _handleSemanticsAction;
     initRenderView();
@@ -143,6 +144,11 @@ abstract class RendererBinding extends BindingBase with SchedulerBinding, Servic
     renderView.configuration = createViewConfiguration();
   }
 
+  /// Called when the platform text scale factor changes.
+  ///
+  /// See [Window.onTextScaleFactorChanged].
+  void handleTextScaleFactorChanged() { }
+
   /// Returns a [ViewConfiguration] configured for the [RenderView] based on the
   /// current environment.
   ///

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

@@ -398,6 +398,15 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
     });
   }
 
+  @override
+  void didChangeTextScaleFactor() {
+    setState(() {
+      // The textScaleFactor property of ui.window has changed. We reference
+      // ui.window in our build function, so we need to call setState(), but
+      // we don't need to cache anything locally.
+    });
+  }
+
   @override
   void didChangeLocale(Locale locale) {
     if (locale == _locale)

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

@@ -90,7 +90,7 @@ abstract class WidgetsBindingObserver {
   /// navigator.
   ///
   /// Observers are expected to return true if they were able to
-  /// handle the notification.  Observers are notified in registration
+  /// handle the notification. Observers are notified in registration
   /// order until one returns true.
   Future<bool> didPushRoute(String route) => new Future<bool>.value(false);
 
@@ -133,7 +133,7 @@ abstract class WidgetsBindingObserver {
   ///
   ///   @override
   ///   Widget build(BuildContext context) {
-  ///     return new Text('Last size: $_lastSize');
+  ///     return new Text('Current size: $_lastSize');
   ///   }
   /// }
   /// ```
@@ -148,6 +148,55 @@ abstract class WidgetsBindingObserver {
   ///    boilerplate.
   void didChangeMetrics() { }
 
+  /// Called when the platform's text scale factor changes.
+  ///
+  /// This typically happens as the result of the user changing system
+  /// preferences, and it should affect all of the text sizes in the
+  /// application.
+  ///
+  /// ## Sample code
+  ///
+  /// ```dart
+  /// class TextScaleFactorReactor extends StatefulWidget {
+  ///   const TextScaleFactorReactor({ Key key }) : super(key: key);
+  ///
+  ///   @override
+  ///   _TextScaleFactorReactorState createState() => new _TextScaleFactorReactorState();
+  /// }
+  ///
+  /// class _TextScaleFactorReactorState extends State<TextScaleFactorReactor> with WidgetsBindingObserver {
+  ///   @override
+  ///   void initState() {
+  ///     super.initState();
+  ///     WidgetsBinding.instance.addObserver(this);
+  ///   }
+  ///
+  ///   @override
+  ///   void dispose() {
+  ///     WidgetsBinding.instance.removeObserver(this);
+  ///     super.dispose();
+  ///   }
+  ///
+  ///   double _lastTextScaleFactor;
+  ///
+  ///   @override
+  ///   void didChangeTextScaleFactor() {
+  ///     setState(() { _lastTextScaleFactor = ui.window.textScaleFactor; });
+  ///   }
+  ///
+  ///   @override
+  ///   Widget build(BuildContext context) {
+  ///     return new Text('Current scale factor: $_lastTextScaleFactor');
+  ///   }
+  /// }
+  /// ```
+  ///
+  /// See also:
+  ///
+  ///  * [MediaQuery.of], which provides a similar service with less
+  ///    boilerplate.
+  void didChangeTextScaleFactor() { }
+
   /// Called when the system tells the app that the user's locale has
   /// changed. For example, if the user changes the system language
   /// settings.
@@ -284,12 +333,6 @@ abstract class WidgetsBinding extends BindingBase with GestureBinding, RendererB
   ///  * [WidgetsBindingObserver], which has an example of using this method.
   bool removeObserver(WidgetsBindingObserver observer) => _observers.remove(observer);
 
-  /// Called when the system metrics change.
-  ///
-  /// Notifies all the observers using
-  /// [WidgetsBindingObserver.didChangeMetrics].
-  ///
-  /// See [Window.onMetricsChanged].
   @override
   void handleMetricsChanged() {
     super.handleMetricsChanged();
@@ -297,6 +340,13 @@ abstract class WidgetsBinding extends BindingBase with GestureBinding, RendererB
       observer.didChangeMetrics();
   }
 
+  @override
+  void handleTextScaleFactorChanged() {
+    super.handleTextScaleFactorChanged();
+    for (WidgetsBindingObserver observer in _observers)
+      observer.didChangeTextScaleFactor();
+  }
+
   /// Called when the system locale changes.
   ///
   /// Calls [dispatchLocaleChanged] to notify the binding observers.

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

@@ -52,7 +52,7 @@ class MediaQueryData {
   MediaQueryData.fromWindow(ui.Window window)
     : size = window.physicalSize / window.devicePixelRatio,
       devicePixelRatio = window.devicePixelRatio,
-      textScaleFactor = 1.0, // TODO(abarth): Read this value from window.
+      textScaleFactor = window.textScaleFactor,
       padding = new EdgeInsets.fromWindowPadding(window.padding, window.devicePixelRatio);
 
   /// The size of the media in logical pixel (e.g, the size of the screen).
@@ -147,7 +147,10 @@ class MediaQueryData {
   int get hashCode => hashValues(size, devicePixelRatio, textScaleFactor, padding);
 
   @override
-  String toString() => '$runtimeType(size: $size, devicePixelRatio: $devicePixelRatio, textScaleFactor: $textScaleFactor, padding: $padding)';
+  String toString() {
+    return '$runtimeType(size: $size, devicePixelRatio: $devicePixelRatio, '
+           'textScaleFactor: $textScaleFactor, padding: $padding)';
+  }
 }
 
 /// Establishes a subtree in which media queries resolve to the given data.

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

@@ -249,7 +249,8 @@ class Text extends StatelessWidget {
   /// For example, if the text scale factor is 1.5, text will be 50% larger than
   /// the specified font size.
   ///
-  /// Defaults to the [MediaQueryData.textScaleFactor] obtained from the ambient
+  /// The value given to the constructor as textScaleFactor.  If null, will
+  /// use the [MediaQueryData.textScaleFactor] obtained from the ambient
   /// [MediaQuery], or 1.0 if there is no [MediaQuery] in scope.
   final double textScaleFactor;
 

packages/flutter/test/material/app_test.dart

@@ -305,4 +305,16 @@ void main() {
     expect(tester.takeException(), isFlutterError);
     expect(log, <String>['onGenerateRoute /', 'onUnknownRoute /']);
   });
+
+  testWidgets('Can get text scale from media query', (WidgetTester tester) async {
+    double textScaleFactor;
+    await tester.pumpWidget(new MaterialApp(
+      home: new Builder(builder:(BuildContext context) {
+        textScaleFactor = MediaQuery.of(context).textScaleFactor;
+        return new Container();
+      }),
+    ));
+    expect(textScaleFactor, isNotNull);
+    expect(textScaleFactor, equals(1.0));
+  });
 }

packages/flutter/test/material/buttons_test.dart

@@ -44,6 +44,46 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('Does FlatButton scale with font scale changes', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Material(
+          child: new MediaQuery(
+            data: const MediaQueryData(textScaleFactor: 1.0),
+            child: new Center(
+              child: new FlatButton(
+                onPressed: () { },
+                child: const Text('ABC'),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byType(FlatButton)), equals(const Size(88.0, 36.0)));
+
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Material(
+          child: new MediaQuery(
+            data: const MediaQueryData(textScaleFactor: 1.3),
+            child: new Center(
+              child: new FlatButton(
+                onPressed: () { },
+                child: const Text('ABC'),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byType(FlatButton)), equals(const Size(88.0, 36.0)));
+  });
+
   // This test is very similar to the '...explicit splashColor and highlightColor' test
   // in icon_button_test.dart. If you change this one, you may want to also change that one.
   testWidgets('MaterialButton with explicit splashColor and highlightColor', (WidgetTester tester) async {

packages/flutter/test/painting/text_painter_test.dart

@@ -57,6 +57,23 @@ void main() {
     expect(painter.size, const Size(123.0, 123.0));
   });
 
+  test('TextPainter textScaleFactor test', () {
+    final TextPainter painter = new TextPainter(
+      text: const TextSpan(
+        text: 'X',
+        style: const TextStyle(
+          inherit: false,
+          fontFamily: 'Ahem',
+          fontSize: 10.0,
+        ),
+      ),
+      textDirection: TextDirection.ltr,
+      textScaleFactor: 2.0,
+    );
+    painter.layout();
+    expect(painter.size, const Size(20.0, 20.0));
+  });
+
   test('TextPainter default text height is 14 pixels', () {
     final TextPainter painter = new TextPainter(
       text: const TextSpan(text: 'x'),

packages/flutter/test/painting/text_style_test.dart

@@ -125,12 +125,12 @@ void main() {
 
   test('TextStyle with text direction', () {
     final ui.ParagraphStyle ps6 = const TextStyle().getParagraphStyle(textDirection: TextDirection.ltr);
-    expect(ps6, equals(new ui.ParagraphStyle(textDirection: TextDirection.ltr)));
-    expect(ps6.toString(), 'ParagraphStyle(textAlign: unspecified, textDirection: TextDirection.ltr, fontWeight: unspecified, fontStyle: unspecified, maxLines: unspecified, fontFamily: unspecified, fontSize: unspecified, lineHeight: unspecified, ellipsis: unspecified)');
+    expect(ps6, equals(new ui.ParagraphStyle(textDirection: TextDirection.ltr, fontSize: 14.0)));
+    expect(ps6.toString(), 'ParagraphStyle(textAlign: unspecified, textDirection: TextDirection.ltr, fontWeight: unspecified, fontStyle: unspecified, maxLines: unspecified, fontFamily: unspecified, fontSize: 14.0, lineHeight: unspecified, ellipsis: unspecified)');
 
     final ui.ParagraphStyle ps7 = const TextStyle().getParagraphStyle(textDirection: TextDirection.rtl);
-    expect(ps7, equals(new ui.ParagraphStyle(textDirection: TextDirection.rtl)));
-    expect(ps7.toString(), 'ParagraphStyle(textAlign: unspecified, textDirection: TextDirection.rtl, fontWeight: unspecified, fontStyle: unspecified, maxLines: unspecified, fontFamily: unspecified, fontSize: unspecified, lineHeight: unspecified, ellipsis: unspecified)');
+    expect(ps7, equals(new ui.ParagraphStyle(textDirection: TextDirection.rtl, fontSize: 14.0)));
+    expect(ps7.toString(), 'ParagraphStyle(textAlign: unspecified, textDirection: TextDirection.rtl, fontWeight: unspecified, fontStyle: unspecified, maxLines: unspecified, fontFamily: unspecified, fontSize: 14.0, lineHeight: unspecified, ellipsis: unspecified)');
   });
 
   test('TextStyle using package font', () {

packages/flutter/test/rendering/paragraph_test.dart

@@ -222,6 +222,64 @@ void main() {
     expect(paragraph.debugNeedsPaint, isFalse);
   });
 
+  test('nested TextSpans in paragraph handle textScaleFactor correctly.', () {
+    final TextSpan testSpan = const TextSpan(
+      text: 'a',
+      style: const TextStyle(
+        fontSize: 10.0,
+      ),
+      children: const <TextSpan>[
+        const TextSpan(
+          text: 'b',
+          children: const <TextSpan>[
+            const TextSpan(text: 'c'),
+          ],
+          style: const TextStyle(
+            fontSize: 20.0,
+          ),
+        ),
+        const TextSpan(
+          text: 'd',
+        ),
+      ],
+    );
+    final RenderParagraph paragraph = new RenderParagraph(
+        testSpan,
+        textDirection: TextDirection.ltr,
+        textScaleFactor: 1.3
+    );
+    paragraph.layout(const BoxConstraints());
+    // anyOf is needed here because Linux and Mac have different text
+    // rendering widths in tests.
+    // TODO(#12357): Figure out why this is, and fix it (if needed) once Blink
+    // text rendering is replaced.
+    expect(paragraph.size.width, anyOf(79.0, 78.0));
+    expect(paragraph.size.height, 26.0);
+
+    // Test the sizes of nested spans.
+    final List<ui.TextBox> boxes = <ui.TextBox>[];
+    final String text = testSpan.toStringDeep();
+    for (int i = 0; i < text.length; ++i) {
+      boxes.addAll(paragraph.getBoxesForSelection(
+          new TextSelection(baseOffset: i, extentOffset: i + 1)
+      ));
+    }
+    expect(boxes.length, equals(4));
+
+    // anyOf is needed here and below because Linux and Mac have different text
+    // rendering widths in tests.
+    // TODO(#12357): Figure out why this is, and fix it (if needed) once Blink
+    // text rendering is replaced.
+    expect(boxes[0].toRect().width, anyOf(14.0, 13.0));
+    expect(boxes[0].toRect().height, 13.0);
+    expect(boxes[1].toRect().width, anyOf(27.0, 26.0));
+    expect(boxes[1].toRect().height, 26.0);
+    expect(boxes[2].toRect().width, anyOf(27.0, 26.0));
+    expect(boxes[2].toRect().height, 26.0);
+    expect(boxes[3].toRect().width, anyOf(14.0, 13.0));
+    expect(boxes[3].toRect().height, 13.0);
+  });
+
   test('toStringDeep', () {
     final RenderParagraph paragraph = new RenderParagraph(
       const TextSpan(text: _kText),

packages/flutter/test/widgets/text_test.dart

@@ -8,7 +8,7 @@ import 'package:flutter/widgets.dart';
 void main() {
   testWidgets('Text respects media query', (WidgetTester tester) async {
     await tester.pumpWidget(const MediaQuery(
-      data: const MediaQueryData(textScaleFactor: 1.5),
+      data: const MediaQueryData(textScaleFactor: 1.3),
       child: const Center(
         child: const Text('Hello', textDirection: TextDirection.ltr)
       )
@@ -16,7 +16,7 @@ void main() {
 
     RichText text = tester.firstWidget(find.byType(RichText));
     expect(text, isNotNull);
-    expect(text.textScaleFactor, 1.5);
+    expect(text.textScaleFactor, 1.3);
 
     await tester.pumpWidget(const Center(
       child: const Text('Hello', textDirection: TextDirection.ltr)
@@ -25,14 +25,58 @@ void main() {
     text = tester.firstWidget(find.byType(RichText));
     expect(text, isNotNull);
     expect(text.textScaleFactor, 1.0);
+  });
+
+  testWidgets('Text respects textScaleFactor with default font size', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Center(child: const Text('Hello', textDirection: TextDirection.ltr))
+    );
+
+    RichText text = tester.firstWidget(find.byType(RichText));
+    expect(text, isNotNull);
+    expect(text.textScaleFactor, 1.0);
+    final Size baseSize = tester.getSize(find.byType(RichText));
+    expect(baseSize.width, equals(70.0));
+    expect(baseSize.height, equals(14.0));
+
+    await tester.pumpWidget(const Center(
+      child: const Text('Hello', textScaleFactor: 1.5, textDirection: TextDirection.ltr)
+    ));
+
+    text = tester.firstWidget(find.byType(RichText));
+    expect(text, isNotNull);
+    expect(text.textScaleFactor, 1.5);
+    final Size largeSize = tester.getSize(find.byType(RichText));
+    expect(largeSize.width, 105.0);
+    expect(largeSize.height, equals(21.0));
+  });
+
+  testWidgets('Text respects textScaleFactor with explicit font size', (WidgetTester tester) async {
+    await tester.pumpWidget(const Center(
+      child: const Text('Hello',
+        style: const TextStyle(fontSize: 20.0), textDirection: TextDirection.ltr)
+    ));
+
+    RichText text = tester.firstWidget(find.byType(RichText));
+    expect(text, isNotNull);
+    expect(text.textScaleFactor, 1.0);
+    final Size baseSize = tester.getSize(find.byType(RichText));
+    expect(baseSize.width, equals(100.0));
+    expect(baseSize.height, equals(20.0));
 
     await tester.pumpWidget(const Center(
-      child: const Text('Hello', textScaleFactor: 3.0, textDirection: TextDirection.ltr)
+      child: const Text('Hello',
+        style: const TextStyle(fontSize: 20.0),
+        textScaleFactor: 1.3,
+        textDirection: TextDirection.ltr)
     ));
 
     text = tester.firstWidget(find.byType(RichText));
     expect(text, isNotNull);
-    expect(text.textScaleFactor, 3.0);
+    expect(text.textScaleFactor, 1.3);
+    final Size largeSize = tester.getSize(find.byType(RichText));
+    expect(largeSize.width, anyOf(131.0, 130.0));
+    expect(largeSize.height, equals(26.0));
   });
 
   testWidgets('Text throws a nice error message if there\'s no Directionality', (WidgetTester tester) async {

packages/flutter_tools/templates/create/android.tmpl/app/src/main/AndroidManifest.xml.tmpl

@@ -20,7 +20,7 @@
             android:name=".MainActivity"
             android:launchMode="singleTop"
             android:theme="@style/LaunchTheme"
-            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
+            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
             <!-- This keeps the window background of the activity showing