fix a scrollbar bug (#95894)

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

@@ -1432,7 +1432,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         case TargetPlatform.linux:
         case TargetPlatform.macOS:
         case TargetPlatform.windows:
-          newPosition = newPosition.clamp(0.0, position.maxScrollExtent);
+          newPosition = newPosition.clamp(position.minScrollExtent, position.maxScrollExtent);
           break;
         case TargetPlatform.iOS:
         case TargetPlatform.android:

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2075,6 +2075,87 @@ void main() {
     await tester.pumpAndSettle();
   });
 
+  testWidgets('Scrollbar thumb can be dragged when the scrollable widget has a negative minScrollExtent', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/95840
+
+    final ScrollController scrollController = ScrollController();
+    final UniqueKey uniqueKey = UniqueKey();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: ScrollConfiguration(
+            behavior: const ScrollBehavior().copyWith(
+              scrollbars: false,
+            ),
+            child: PrimaryScrollController(
+              controller: scrollController,
+              child: RawScrollbar(
+                isAlwaysShown: true,
+                controller: scrollController,
+                child: CustomScrollView(
+                  center: uniqueKey,
+                  slivers: <Widget>[
+                    SliverToBoxAdapter(
+                      child: Container(
+                        height: 600.0,
+                      ),
+                    ),
+                    SliverToBoxAdapter(
+                      key: uniqueKey,
+                      child: Container(
+                        height: 600.0,
+                      ),
+                    ),
+                    SliverToBoxAdapter(
+                      child: Container(
+                        height: 600.0,
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 200.0, 800.0, 400.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Drag the thumb up to scroll up.
+    const double scrollAmount = -10.0;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 300.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(scrollController.offset, lessThan(scrollAmount * 2));
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 190.0, 800.0, 390.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  }, variant: TargetPlatformVariant.all());
+
   test('ScrollbarPainter.shouldRepaint returns true when any of the properties changes', () {
     ScrollbarPainter createPainter({
       Color color = const Color(0xFF000000),