Update the requirements for applying the elevation overlay. (#57526)

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

@@ -14,19 +14,19 @@ import 'theme.dart';
 /// This is an internal implementation class and should not be exported by
 /// the material package.
 class ElevationOverlay {
-  // This class is not meant to be instatiated or extended; this constructor
+  // This class is not meant to be instantiated or extended; this constructor
   // prevents instantiation and extension.
   // ignore: unused_element
   ElevationOverlay._();
 
-  /// Applies an elevation overlay color to a surface color to indicate
+  /// Applies an elevation overlay color to a given color to indicate
   /// the level of elevation in a dark theme.
   ///
-  /// If the surrounding [Theme.applyElevationOverlayColor] is true, and
-  /// [color] is [Theme.colorScheme.surface] then this will return
-  /// a version of the given color with a semi-transparent [Theme.colorScheme.onSurface]
-  /// overlaid on top of it. The opacity of the overlay is controlled by the
-  /// [elevation].
+  /// If the ambient [ThemeData.applyElevationOverlayColor] is true,
+  /// and [ThemeData.brightness] is [Brightness.dark] then this will return
+  /// a version of the given color with a semi-transparent
+  /// [ThemeData.colorScheme.onSurface] overlaid on top of it. The opacity
+  /// of the overlay is computed based on the [elevation].
   ///
   /// Otherwise it will just return the [color] unmodified.
   ///
@@ -39,7 +39,7 @@ class ElevationOverlay {
     final ThemeData theme = Theme.of(context);
     if (elevation > 0.0 &&
         theme.applyElevationOverlayColor &&
-        color == theme.colorScheme.surface) {
+        theme.brightness == Brightness.dark) {
 
       return Color.alphaBlend(overlayColor(context, elevation), color);
     }

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

@@ -227,9 +227,9 @@ class Material extends StatefulWidget {
   /// [MaterialType.transparency].
   ///
   /// To support dark themes, if the surrounding
-  /// [ThemeData.applyElevationOverlayColor] is true then a semi-transparent
-  /// overlay color will be composited on top this color to indicate
-  /// the elevation.
+  /// [ThemeData.applyElevationOverlayColor] is true and [ThemeData.brightness]
+  /// is [Brightness.dark] then a semi-transparent overlay color will be
+  /// composited on top of this color to indicate the elevation.
   ///
   /// By default, the color is derived from the [type] of material.
   final Color color;

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

@@ -933,12 +933,13 @@ class ThemeData with Diagnosticable {
   /// elevation of a surface should be portrayed with an "overlay" in addition
   /// to the shadow. As the elevation of the component increases, the
   /// overlay increases in opacity. [applyElevationOverlayColor] turns the
-  /// application of this overlay on or off.
+  /// application of this overlay on or off for dark themes.
   ///
-  /// If [true] a semi-transparent version of [colorScheme.onSurface] will be
-  /// applied on top of the color of [Material] widgets when their [Material.color]
-  /// is [colorScheme.surface]. The level of transparency is based on
-  /// [Material.elevation] as per the Material Dark theme specification.
+  /// If [true] and [brightness] is [Brightness.dark], a
+  /// semi-transparent version of [colorScheme.onSurface] will be
+  /// applied on top of the color of [Material] widgets. The level of
+  /// transparency is based on [Material.elevation] as per the
+  /// Material Dark theme specification.
   ///
   /// If [false] the surface color will be used unmodified.
   ///
@@ -951,9 +952,9 @@ class ThemeData with Diagnosticable {
   ///
   /// See also:
   ///
-  ///  * [Material.elevation], which effects how transparent the white overlay is.
-  ///  * [Material.color], the white color overlay will only be applied of the
-  ///    material's color is [colorScheme.surface].
+  ///  * [Material.elevation], which effects the level of transparency of the
+  ///    overlay color.
+  ///  * [Material.color], the elevation overlay will be applied to this color.
   ///  * <https://material.io/design/color/dark-theme.html>, which specifies how
   ///    the overlay should be applied.
   final bool applyElevationOverlayColor;

packages/flutter/test/material/material_test.dart

@@ -273,20 +273,21 @@ void main() {
       }
     });
 
-    testWidgets('overlay will only apply to materials using colorScheme.surface', (WidgetTester tester) async {
+    testWidgets('overlay will not apply to materials using a light theme', (WidgetTester tester) async {
       await tester.pumpWidget(
           Theme(
             data: ThemeData(
               applyElevationOverlayColor: true,
-              colorScheme: const ColorScheme.dark().copyWith(surface: const Color(0xFF121212)),
+              colorScheme: const ColorScheme.light(),
             ),
             child: buildMaterial(
-                color: Colors.cyan,
-                elevation: 8.0,
+              color: Colors.cyan,
+              elevation: 8.0,
             ),
           ),
       );
       final RenderPhysicalShape model = getModel(tester);
+      // Shouldn't change, as it was under a light color scheme.
       expect(model.color, equals(Colors.cyan));
     });