add tests for dominant bottom sheet in scaffold (#124472)

while debugging https://github.com/flutter/flutter/issues/123882 found out that below lines were untested. although they'll change due to the issue above, but the tests will remain.

https://github.com/flutter/flutter/blob/e0045d04f7057d91b1701a444f75f2f0458c1d1a/packages/flutter/lib/src/material/scaffold.dart#L3169-L3180

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

@@ -1436,8 +1436,6 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// within the [Scaffold]. The [FloatingActionButton] is connected to a
 /// callback that increments a counter.
 ///
-/// ![The Scaffold has a white background with a blue AppBar at the top. A blue FloatingActionButton is positioned at the bottom right corner of the Scaffold.](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold.png)
-///
 /// ** See code in examples/api/lib/material/scaffold/scaffold.0.dart **
 /// {@end-tool}
 ///
@@ -1485,6 +1483,19 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// widget can be used within the scaffold's body to avoid areas
 /// like display notches.
 ///
+/// ## Floating action button with a draggable scrollable bottom sheet
+///
+/// If [Scaffold.bottomSheet] is a [DraggableScrollableSheet],
+/// [Scaffold.floatingActionButton] is set, and the bottom sheet is dragged to
+/// cover greater than 70% of the Scaffold's height, two things happen in parallel:
+///
+///   * Scaffold starts to show scrim (see [ScaffoldState.showBodyScrim]), and
+///   * [Scaffold.floatingActionButton] is scaled down through an animation with a [Curves.easeIn], and
+///     disappears when the bottom sheet covers the entire Scaffold.
+///
+/// And as soon as the bottom sheet is dragged down to cover less than 70% of the [Scaffold], the scrim
+/// disappears and [Scaffold.floatingActionButton] animates back to its normal size.
+///
 /// ## Troubleshooting
 ///
 /// ### Nested Scaffolds

packages/flutter/test/material/scaffold_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart' show DragStartBehavior;
 import 'package:flutter/material.dart';
@@ -266,6 +268,84 @@ void main() {
     expect(tester.binding.transientCallbackCount, greaterThan(0));
   });
 
+  testWidgets('Floating action button shrinks when bottom sheet becomes dominant', (WidgetTester tester) async {
+    final DraggableScrollableController draggableController = DraggableScrollableController();
+    const double kBottomSheetDominatesPercentage = 0.3;
+
+    await tester.pumpWidget(MaterialApp(home: Scaffold(
+      floatingActionButton: const FloatingActionButton(
+        key: Key('one'),
+        onPressed: null,
+        child: Text('1'),
+      ),
+      bottomSheet: DraggableScrollableSheet(
+        expand: false,
+        controller: draggableController,
+        builder: (BuildContext context, ScrollController scrollController) {
+          return SingleChildScrollView(
+            controller: scrollController,
+            child: const SizedBox(),
+          );
+        },
+      ),
+    )));
+
+    double getScale() => tester.firstWidget<ScaleTransition>(find.byType(ScaleTransition)).scale.value;
+
+    for (double i = 0, extent = i / 10; i <= 10; i++, extent = i / 10) {
+      draggableController.jumpTo(extent);
+
+      final double extentRemaining = 1.0 - extent;
+      if (extentRemaining < kBottomSheetDominatesPercentage) {
+        final double visValue = extentRemaining * kBottomSheetDominatesPercentage * 10;
+        // since FAB uses easeIn curve, we're testing this by using the fact that
+        // easeIn curve is always less than or equal to x=y curve.
+        expect(getScale(), lessThanOrEqualTo(visValue));
+      } else {
+        expect(getScale(), equals(1.0));
+      }
+    }
+  });
+
+  testWidgets('Scaffold shows scrim when bottom sheet becomes dominant', (WidgetTester tester) async {
+    final DraggableScrollableController draggableController = DraggableScrollableController();
+    const double kBottomSheetDominatesPercentage = 0.3;
+    const double kMinBottomSheetScrimOpacity = 0.1;
+    const double kMaxBottomSheetScrimOpacity = 0.6;
+
+    await tester.pumpWidget(MaterialApp(home: Scaffold(
+      bottomSheet: DraggableScrollableSheet(
+        expand: false,
+        controller: draggableController,
+        builder: (BuildContext context, ScrollController scrollController) {
+          return SingleChildScrollView(
+            controller: scrollController,
+            child: const SizedBox(),
+          );
+        },
+      ),
+    )));
+
+    Finder findModalBarrier() => find.descendant(of: find.byType(Scaffold), matching: find.byType(ModalBarrier));
+    double getOpacity() => tester.firstWidget<ModalBarrier>(findModalBarrier()).color!.opacity;
+    double getExpectedOpacity(double visValue) => math.max(kMinBottomSheetScrimOpacity, kMaxBottomSheetScrimOpacity - visValue);
+
+    for (double i = 0, extent = i / 10; i <= 10; i++, extent = i / 10) {
+      draggableController.jumpTo(extent);
+      await tester.pump();
+
+      final double extentRemaining = 1.0 - extent;
+      if (extentRemaining < kBottomSheetDominatesPercentage) {
+        final double visValue = extentRemaining * kBottomSheetDominatesPercentage * 10;
+
+        expect(findModalBarrier(), findsOneWidget);
+        expect(getOpacity(), moreOrLessEquals(getExpectedOpacity(visValue), epsilon: 0.02));
+      } else {
+        expect(findModalBarrier(), findsNothing);
+      }
+    }
+  });
+
   testWidgets('Floating action button directionality', (WidgetTester tester) async {
     Widget build(TextDirection textDirection) {
       return Directionality(