Ensure that animated pairs of Tabs TextStyles have matching inherited values (#29175)

packages/flutter/lib/src/material/tabs.dart

@@ -153,16 +153,23 @@ class _TabStyle extends AnimatedWidget {
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context);
     final TabBarTheme tabBarTheme = TabBarTheme.of(context);
+    final Animation<double> animation = listenable;
 
-    final TextStyle defaultStyle = labelStyle ?? tabBarTheme.labelStyle ?? themeData.primaryTextTheme.body2;
-    final TextStyle defaultUnselectedStyle = unselectedLabelStyle
+    // To enable TextStyle.lerp(style1, style2, value), both styles must have
+    // the same value of inherit. Force that to be inherit=true here.
+    final TextStyle defaultStyle = (labelStyle
+      ?? tabBarTheme.labelStyle
+      ?? themeData.primaryTextTheme.body2
+    ).copyWith(inherit: true);
+    final TextStyle defaultUnselectedStyle = (unselectedLabelStyle
       ?? tabBarTheme.unselectedLabelStyle
       ?? labelStyle
-      ?? themeData.primaryTextTheme.body2;
-    final Animation<double> animation = listenable;
+      ?? themeData.primaryTextTheme.body2
+    ).copyWith(inherit: true);
     final TextStyle textStyle = selected
       ? TextStyle.lerp(defaultStyle, defaultUnselectedStyle, animation.value)
       : TextStyle.lerp(defaultUnselectedStyle, defaultStyle, animation.value);
+
     final Color selectedColor = labelColor
        ?? tabBarTheme.labelColor
        ?? themeData.primaryTextTheme.body2.color;

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

@@ -522,6 +522,7 @@ class TextStyle extends Diagnosticable {
   /// [background] specified it will be given preference over any
   /// backgroundColor parameter.
   TextStyle copyWith({
+    bool inherit,
     Color color,
     Color backgroundColor,
     String fontFamily,
@@ -551,7 +552,7 @@ class TextStyle extends Diagnosticable {
       return true;
     }());
     return TextStyle(
-      inherit: inherit,
+      inherit: inherit ?? this.inherit,
       color: this.foreground == null && foreground == null ? color ?? this.color : null,
       backgroundColor: this.background == null && background == null ? backgroundColor ?? this.backgroundColor : null,
       fontFamily: fontFamily ?? this.fontFamily,

packages/flutter/test/material/tab_bar_theme_test.dart

@@ -82,6 +82,23 @@ void main() {
     expect(unselectedRenderObject.text.style.fontFamily, equals(unselectedLabelStyle.fontFamily));
   });
 
+  testWidgets('Tab bar theme with just label style specified', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/28784
+    const TextStyle labelStyle = TextStyle(fontFamily: 'foobar');
+    const TabBarTheme tabBarTheme = TabBarTheme(
+      labelStyle: labelStyle,
+    );
+
+    await tester.pumpWidget(_withTheme(tabBarTheme));
+
+    final RenderParagraph selectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab1Text));
+    expect(selectedRenderObject.text.style.fontFamily, equals(labelStyle.fontFamily));
+    final RenderParagraph unselectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab2Text));
+    expect(unselectedRenderObject.text.style.fontFamily, equals('Roboto'));
+    expect(unselectedRenderObject.text.style.fontSize, equals(14.0));
+    expect(unselectedRenderObject.text.style.color, equals(Colors.white.withAlpha(0xB2)));
+  });
+
   testWidgets('Tab bar label styles override theme label styles', (WidgetTester tester) async {
     const TextStyle labelStyle = TextStyle(fontFamily: '1');
     const TextStyle unselectedLabelStyle = TextStyle(fontFamily: '2');

packages/flutter/test/material/theme_test.dart

@@ -731,7 +731,7 @@ class _TextStyleProxy implements TextStyle {
   }
 
   @override
-  TextStyle copyWith({ Color color, Color backgroundColor, String fontFamily, List<String> fontFamilyFallback, double fontSize, FontWeight fontWeight, FontStyle fontStyle, double letterSpacing, double wordSpacing, TextBaseline textBaseline, double height, Locale locale, ui.Paint foreground, ui.Paint background, List<Shadow> shadows, TextDecoration decoration, Color decorationColor, TextDecorationStyle decorationStyle, String debugLabel }) {
+  TextStyle copyWith({ bool inherit, Color color, Color backgroundColor, String fontFamily, List<String> fontFamilyFallback, double fontSize, FontWeight fontWeight, FontStyle fontStyle, double letterSpacing, double wordSpacing, TextBaseline textBaseline, double height, Locale locale, ui.Paint foreground, ui.Paint background, List<Shadow> shadows, TextDecoration decoration, Color decorationColor, TextDecorationStyle decorationStyle, String debugLabel }) {
     throw UnimplementedError();
   }
 

packages/flutter/test/painting/text_style_test.dart

@@ -33,6 +33,12 @@ void main() {
       equals('TextStyle(inherit: true, size: 10.0, weight: 800, height: 123.0x)'),
     );
 
+    // Check that the inherit flag can be set with copyWith().
+    expect(
+      s1.copyWith(inherit: false).toString(),
+      equals('TextStyle(inherit: false, size: 10.0, weight: 800, height: 123.0x)'),
+    );
+
     final TextStyle s2 = s1.copyWith(color: const Color(0xFF00FF00), height: 100.0);
     expect(s1.fontFamily, isNull);
     expect(s1.fontSize, 10.0);