Update OutlineButton default border width and highlight elevation (#27575)

These changes are **backwards incompatible**. Tests that verify OutlineButton visuals, for example golden image tests, will need to be updated.

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

@@ -632,16 +632,15 @@ class ButtonThemeData extends Diagnosticable {
   ///
   /// Returns the button's [MaterialButton.highlightElevation] if it is non-null.
   ///
-  /// If button is a [FlatButton] then the highlight elevation is 0.0, if it's
-  /// a [OutlineButton] then the highlight elevation is 2.0, otherise the
-  /// highlight elevation is 8.0.
+  /// If button is a [FlatButton] or an [OutlineButton] then the highlight
+  /// elevation is 0.0, otherise the highlight elevation is 8.0.
   double getHighlightElevation(MaterialButton button) {
     if (button.highlightElevation != null)
       return button.highlightElevation;
     if (button is FlatButton)
       return 0.0;
     if (button is OutlineButton)
-      return 2.0;
+      return 0.0;
     return 8.0;
   }
 

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

@@ -12,28 +12,31 @@ import 'raised_button.dart';
 import 'theme.dart';
 
 // The total time to make the button's fill color opaque and change
-// its elevation.
+// its elevation. Only applies when highlightElevation > 0.0.
 const Duration _kPressDuration = Duration(milliseconds: 150);
 
 // Half of _kPressDuration: just the time to change the button's
-// elevation.
+// elevation. Only applies when highlightElevation > 0.0.
 const Duration _kElevationDuration = Duration(milliseconds: 75);
 
-/// A cross between [RaisedButton] and [FlatButton]: a bordered button whose
-/// elevation increases and whose background becomes opaque when the button
-/// is pressed.
+/// Similar to a [FlatButton] with a thin grey rounded rectangle border.
 ///
-/// An outline button's elevation is initially 0.0 and its background [color]
-/// is transparent. When the button is pressed its background becomes opaque
-/// and then its elevation increases to [highlightElevation].
-///
-/// The outline button has a border whose shape is defined by [shape]
-/// and whose appearance is defined by [borderSide], [disabledBorderColor],
-/// and [highlightedBorderColor].
+/// The outline button's border shape is defined by [shape]
+/// and its appearance is defined by [borderSide], [disabledBorderColor],
+/// and [highlightedBorderColor]. By default the border is a one pixel
+/// wide grey rounded rectangle that does not change when the button is
+/// pressed or disabled. By default the button's background is transparent.
 ///
 /// If the [onPressed] callback is null, then the button will be disabled and by
 /// default will resemble a flat button in the [disabledColor].
 ///
+/// The button's [highlightElevation], which defines the size of the
+/// drop shadow when the button is pressed, is 0.0 (no shadow) by default.
+/// If [highlightElevation] is given a value greater than 0.0 then the button
+/// becomes a cross between [RaisedButton] and [FlatButton]: a bordered
+/// button whose elevation increases and whose background becomes opaque
+/// when the button is pressed.
+///
 /// If you want an ink-splash effect for taps, but don't want to use a button,
 /// consider using [InkWell] directly.
 ///
@@ -50,10 +53,10 @@ const Duration _kElevationDuration = Duration(milliseconds: 75);
 ///  * [InkWell], which implements the ink splash part of a flat button.
 ///  * <https://material.io/design/components/buttons.html>
 class OutlineButton extends MaterialButton {
-  /// Create a filled button.
+  /// Create an outline button.
   ///
-  /// The [highlightElevation], [borderWidth], and [clipBehavior]
-  /// arguments must not be null.
+  /// The [highlightElevation] argument must be null or a positive value
+  /// and the [clipBehavior] argument must not be null.
   const OutlineButton({
     Key key,
     @required VoidCallback onPressed,
@@ -94,8 +97,8 @@ class OutlineButton extends MaterialButton {
   /// The icon and label are arranged in a row and padded by 12 logical pixels
   /// at the start, and 16 at the end, with an 8 pixel gap in between.
   ///
-  /// The [highlightElevation], [icon], [label], and [clipBehavior] must not be
-  /// null.
+  /// The [highlightElevation] argument must be null or a positive value. The
+  /// [icon], [label], and [clipBehavior] arguments must not be null.
   factory OutlineButton.icon({
     Key key,
     @required VoidCallback onPressed,
@@ -118,15 +121,14 @@ class OutlineButton extends MaterialButton {
 
   /// The outline border's color when the button is [enabled] and pressed.
   ///
-  /// If null this value defaults to the theme's primary color,
-  /// [ThemeData.primaryColor].
+  /// By default the border's color does not change when the button
+  /// is pressed.
   final Color highlightedBorderColor;
 
   /// The outline border's color when the button is not [enabled].
   ///
-  /// If null this value defaults to a very light shade of grey for light
-  /// themes (see [ThemeData.brightness]), and a very dark shade of grey for
-  /// dark themes.
+  /// By default the outline border's color does not change when the
+  /// button is disabled.
   final Color disabledBorderColor;
 
   /// Defines the color of the border when the button is enabled but not
@@ -136,7 +138,7 @@ class OutlineButton extends MaterialButton {
   /// an outline is not drawn.
   ///
   /// If null the default border's style is [BorderStyle.solid], its
-  /// [BorderSide.width] is 2.0, and its color is a light shade of grey.
+  /// [BorderSide.width] is 1.0, and its color is a light shade of grey.
   final BorderSide borderSide;
 
   @override
@@ -181,10 +183,10 @@ class OutlineButton extends MaterialButton {
   }
 }
 
-// The type of of OutlineButtons created with [OutlineButton.icon].
+// The type of of OutlineButtons created with OutlineButton.icon.
 //
-// This class only exists to give RaisedButtons created with [RaisedButton.icon]
-// a distinct class for the sake of [ButtonTheme]. It can not be instantiated.
+// This class only exists to give OutlineButtons created with OutlineButton.icon
+// a distinct class for the sake of ButtonTheme. It can not be instantiated.
 class _OutlineButtonWithIcon extends OutlineButton with MaterialButtonWithIconMixin {
   _OutlineButtonWithIcon({
     Key key,
@@ -291,12 +293,13 @@ class _OutlineButtonState extends State<_OutlineButton> with SingleTickerProvide
   void initState() {
     super.initState();
 
-    // The Material widget animates its shape (which includes the outline
-    // border) and elevation over _kElevationDuration. When pressed, the
-    // button makes its fill color opaque white first, and then sets
-    // its highlightElevation. We can't change the elevation while the
-    // button's fill is translucent, because the shadow fills the interior
-    // of the button.
+    // When highlightElevation > 0.0, the Material widget animates its
+    // shape (which includes the outline border) and elevation over
+    // _kElevationDuration. When pressed, the button makes its fill
+    // color opaque white first, and then sets its
+    // highlightElevation. We can't change the elevation while the
+    // button's fill is translucent, because the shadow fills the
+    // interior of the button.
 
     _controller = AnimationController(
       duration: _kPressDuration,
@@ -343,6 +346,8 @@ class _OutlineButtonState extends State<_OutlineButton> with SingleTickerProvide
   }
 
   Color _getFillColor() {
+    if (widget.highlightElevation == null || widget.highlightElevation == 0.0)
+      return Colors.transparent;
     final Color color = widget.color ?? Theme.of(context).canvasColor;
     final Tween<Color> colorTween = ColorTween(
       begin: color.withAlpha(0x00),
@@ -352,28 +357,27 @@ class _OutlineButtonState extends State<_OutlineButton> with SingleTickerProvide
   }
 
   BorderSide _getOutline() {
-    final bool isDark = widget.brightness == Brightness.dark;
     if (widget.borderSide?.style == BorderStyle.none)
       return widget.borderSide;
 
-    final Color color = widget.enabled
-      ? (_pressed
-         ? widget.highlightedBorderColor
-         : (widget.borderSide?.color ??
-            (isDark ? Colors.grey[600] : Colors.grey[200])))
-      : (widget.disabledBorderColor ??
-         (isDark ? Colors.grey[800] : Colors.grey[100]));
+    final Color specifiedColor = widget.enabled
+      ? (_pressed ? widget.highlightedBorderColor : null) ?? widget.borderSide?.color
+      : widget.disabledBorderColor;
+
+    final Color themeColor = Theme.of(context).colorScheme.onSurface.withOpacity(0.12);
 
     return BorderSide(
-      color: color,
-      width: widget.borderSide?.width ?? 2.0,
+      color: specifiedColor ?? themeColor,
+      width: widget.borderSide?.width ?? 1.0,
     );
   }
 
   double _getHighlightElevation() {
+    if (widget.highlightElevation == null || widget.highlightElevation == 0.0)
+      return 0.0;
     return Tween<double>(
       begin: 0.0,
-      end: widget.highlightElevation ?? 2.0,
+      end: widget.highlightElevation,
     ).evaluate(_elevationAnimation);
   }
 

packages/flutter/test/material/outline_button_test.dart

@@ -81,6 +81,10 @@ void main() {
               shape: const RoundedRectangleBorder(), // default border radius is 0
               clipBehavior: Clip.antiAlias,
               color: fillColor,
+              // Causes the button to be filled with the theme's canvasColor
+              // instead of Colors.transparent before the button material's
+              // elevation is animated to 2.0.
+              highlightElevation: 2.0,
               highlightedBorderColor: highlightedBorderColor,
               disabledBorderColor: disabledBorderColor,
               borderSide: const BorderSide(
@@ -108,7 +112,7 @@ void main() {
     // Expect that the button is disabled and painted with the disabled border color.
     expect(tester.widget<OutlineButton>(outlineButton).enabled, false);
     expect(
-      outlineButton, //find.byType(OutlineButton),
+      outlineButton,
       paints
         ..clipPath(pathMatcher: coversSameAreaAs(clipPath, areaToCompare: clipRect.inflate(10.0)))
         ..path(color: disabledBorderColor, strokeWidth: borderWidth));
@@ -323,6 +327,10 @@ void main() {
           body: Center(
             child: OutlineButton(
               onPressed: () {},
+              // Causes the button to be filled with the theme's canvasColor
+              // instead of Colors.transparent before the button material's
+              // elevation is animated to 2.0.
+              highlightElevation: 2.0,
               child: const Text('Hello'),
             ),
           ),

packages/flutter/test/material/theme_defaults_test.dart

@@ -128,13 +128,17 @@ void main() {
   });
 
   group('OutlineButton', () {
-    testWidgets('theme: ThemeData.light(), enabled: true', (WidgetTester tester) async {
+    testWidgets('theme: ThemeData.light(), enabled: true, highlightElevation: 2.0', (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(
           theme: ThemeData.light(),
           home: Center(
             child: OutlineButton(
               onPressed: () { }, // button.enabled == true
+              // Causes the button to be filled with the theme's canvasColor
+              // instead of Colors.transparent before the button material's
+              // elevation is animated to 2.0.
+              highlightElevation: 2.0,
               child: const Text('button'),
             )
           ),
@@ -155,6 +159,33 @@ void main() {
       expect(raw.materialTapTargetSize, MaterialTapTargetSize.padded);
     });
 
+    testWidgets('theme: ThemeData.light(), enabled: true', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          theme: ThemeData.light(),
+          home: Center(
+            child: OutlineButton(
+              onPressed: () { }, // button.enabled == true
+              child: const Text('button'),
+            )
+          ),
+        ),
+      );
+
+      final RawMaterialButton raw = tester.widget<RawMaterialButton>(find.byType(RawMaterialButton));
+      expect(raw.textStyle.color, const Color(0xdd000000));
+      expect(raw.fillColor, Colors.transparent);
+      expect(raw.highlightColor, const Color(0x29000000)); // Was Color(0x66bcbcbc)
+      expect(raw.splashColor, const Color(0x1f000000)); // Was Color(0x66c8c8c8)
+      expect(raw.elevation, 0.0);
+      expect(raw.highlightElevation, 0.0);
+      expect(raw.disabledElevation, 0.0);
+      expect(raw.constraints, defaultButtonConstraints);
+      expect(raw.padding, defaultButtonPadding);
+      // animationDuration can't be configed by the theme/constructor
+      expect(raw.materialTapTargetSize, MaterialTapTargetSize.padded);
+    });
+
     testWidgets('theme: ThemeData.light(), enabled: false', (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(