Fixing constraints.precedingScrollExtent passed to SliverPadding child (#49433)

packages/flutter/lib/src/rendering/sliver_padding.dart

@@ -139,6 +139,7 @@ abstract class RenderSliverEdgeInsetsPadding extends RenderSliver with RenderObj
         remainingPaintExtent: constraints.remainingPaintExtent - calculatePaintOffset(constraints, from: 0.0, to: beforePadding),
         remainingCacheExtent: constraints.remainingCacheExtent - calculateCacheOffset(constraints, from: 0.0, to: beforePadding),
         crossAxisExtent: math.max(0.0, constraints.crossAxisExtent - crossAxisPadding),
+        precedingScrollExtent: beforePadding + constraints.precedingScrollExtent,
       ),
       parentUsesSize: true,
     );

packages/flutter/test/widgets/slivers_padding_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -423,4 +424,37 @@ void main() {
       const Rect.fromLTRB(0.0, -200.0, 800.0, 200.0),
     );
   });
+
+  testWidgets('SliverPadding includes preceding padding in the precedingScrollExtent provided to child', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/49195
+    final UniqueKey key = UniqueKey();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        slivers: <Widget>[
+          SliverPadding(
+            padding: const EdgeInsets.only(top: 30),
+            sliver: SliverFillRemaining(
+              hasScrollBody: false,
+              child: Container(
+                key: key,
+                color: Colors.red,
+              ),
+            )
+          ),
+        ]
+      ),
+    ));
+    await tester.pump();
+
+    // The value of 570 is expected since SliverFillRemaining will fill all of
+    // the space available to it. In this test, the extent of the viewport is
+    // 600 pixels. If the SliverPadding widget provides the right constraints
+    // to SliverFillRemaining, with 30 pixels preceding it, it should only have
+    // a height of 570.
+    expect(
+      tester.renderObject<RenderBox>(find.byKey(key)).size.height,
+      equals(570),
+    );
+  });
 }