Allow DSS to be dragged when its children do not fill extent (#31832)

* Allow DSS to be dragged when its children do not fill extent

* Fix when maxChildSize  < 1.0

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

@@ -262,13 +262,16 @@ class _DraggableSheetExtent {
   }
   double get currentExtent => _currentExtent.value;
 
+  double get additionalMinExtent => isAtMin ? 0.0 : 1.0;
+  double get additionalMaxExtent => isAtMax ? 0.0 : 1.0;
+
   /// The scroll position gets inputs in terms of pixels, but the extent is
   /// expected to be expressed as a number between 0..1.
   void addPixelDelta(double delta, BuildContext context) {
     if (availablePixels == 0) {
       return;
     }
-    currentExtent += delta / availablePixels;
+    currentExtent += delta / availablePixels * maxExtent;
     DraggableScrollableNotification(
       minExtent: minExtent,
       maxExtent: maxExtent,
@@ -426,6 +429,17 @@ class _DraggableScrollableSheetScrollPosition
   final _DraggableSheetExtent extent;
   bool get listShouldScroll => pixels > 0.0;
 
+  @override
+  bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
+    // We need to provide some extra extent if we haven't yet reached the max or
+    // min extents. Otherwise, a list with fewer children than the extent of
+    // the available space will get stuck.
+    return super.applyContentDimensions(
+      minScrollExtent - extent.additionalMinExtent,
+      maxScrollExtent + extent.additionalMaxExtent,
+    );
+  }
+
   @override
   void applyUserOffset(double delta) {
     if (!listShouldScroll &&

packages/flutter/test/material/modal_bottom_sheet_test.dart

@@ -297,6 +297,7 @@ void main() {
               children: <TestSemantics>[
                 TestSemantics(
                   flags: <SemanticsFlag>[SemanticsFlag.hasImplicitScrolling],
+                  actions: <SemanticsAction>[SemanticsAction.scrollDown, SemanticsAction.scrollUp],
                   children: <TestSemantics>[
                     TestSemantics(
                       label: 'BottomSheet',

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -8,7 +8,14 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  Widget _boilerplate(VoidCallback onButtonPressed) {
+  Widget _boilerplate(VoidCallback onButtonPressed, {
+    int itemCount = 100,
+    double initialChildSize = .5,
+    double maxChildSize = 1.0,
+    double minChildSize = .25,
+    double itemExtent,
+    Key containerKey,
+  }) {
     return Directionality(
       textDirection: TextDirection.ltr,
       child: Stack(
@@ -18,14 +25,17 @@ void main() {
             onPressed: onButtonPressed,
           ),
           DraggableScrollableSheet(
-            maxChildSize: 1.0,
+            maxChildSize: maxChildSize,
-            minChildSize: .25,
+            minChildSize: minChildSize,
+            initialChildSize: initialChildSize,
             builder: (BuildContext context, ScrollController scrollController) {
               return Container(
+                key: containerKey,
                 color: const Color(0xFFABCDEF),
                 child: ListView.builder(
                   controller: scrollController,
-                  itemCount: 100,
+                  itemExtent: itemExtent,
+                  itemCount: itemCount,
                   itemBuilder: (BuildContext context, int index) => Text('Item $index'),
                 ),
               );
@@ -36,6 +46,38 @@ void main() {
     );
   }
 
+  testWidgets('Scrolls correct amount when maxChildSize < 1.0', (WidgetTester tester) async {
+    const Key key = ValueKey<String>('container');
+    await tester.pumpWidget(_boilerplate(
+      null,
+      maxChildSize: .6,
+      initialChildSize: .25,
+      itemExtent: 25.0,
+      containerKey: key,
+    ));
+
+    expect(tester.getRect(find.byKey(key)), const Rect.fromLTRB(0.0, 450.0, 800.0, 600.0));
+    await tester.drag(find.text('Item 5'), const Offset(0, -125));
+    await tester.pumpAndSettle();
+    expect(tester.getRect(find.byKey(key)), const Rect.fromLTRB(0.0, 325.0, 800.0, 600.0));
+  });
+
+  testWidgets('Scrolls correct amount when maxChildSize == 1.0', (WidgetTester tester) async {
+    const Key key = ValueKey<String>('container');
+    await tester.pumpWidget(_boilerplate(
+      null,
+      maxChildSize: 1.0,
+      initialChildSize: .25,
+      itemExtent: 25.0,
+      containerKey: key,
+    ));
+
+    expect(tester.getRect(find.byKey(key)), const Rect.fromLTRB(0.0, 450.0, 800.0, 600.0));
+    await tester.drag(find.text('Item 5'), const Offset(0, -125));
+    await tester.pumpAndSettle();
+    expect(tester.getRect(find.byKey(key)), const Rect.fromLTRB(0.0, 325.0, 800.0, 600.0));
+  });
+
   for (TargetPlatform platform in TargetPlatform.values) {
     group('$platform Scroll Physics', () {
       debugDefaultTargetPlatformOverride = platform;
@@ -74,6 +116,23 @@ void main() {
         expect(find.text('Item 36'), findsNothing);
       });
 
+      testWidgets('Can be dragged down when list is shorter than full height', (WidgetTester tester) async {
+        await tester.pumpWidget(_boilerplate(null, itemCount: 30, initialChildSize: .25));
+
+        expect(find.text('Item 1').hitTestable(), findsOneWidget);
+        expect(find.text('Item 29').hitTestable(), findsNothing);
+
+        await tester.drag(find.text('Item 1'), const Offset(0, -325));
+        await tester.pumpAndSettle();
+        expect(find.text('Item 1').hitTestable(), findsOneWidget);
+        expect(find.text('Item 29').hitTestable(), findsOneWidget);
+
+        await tester.drag(find.text('Item 1'), const Offset(0, 325));
+        await tester.pumpAndSettle();
+        expect(find.text('Item 1').hitTestable(), findsOneWidget);
+        expect(find.text('Item 29').hitTestable(), findsNothing);
+      });
+
       testWidgets('Can be dragged up and cover its container and scroll in single motion, and then dragged back down', (WidgetTester tester) async {
         int taps = 0;
         await tester.pumpWidget(_boilerplate(() => taps++));