SliverFillRemaining accounts for child size when hasScrollBody is false (#35810)

Fixes the hasScrollBody flag not accounting for child size. Adds the ability to specify over-scroll behavior.

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

@@ -7,6 +7,7 @@ import 'dart:math' as math;
 import 'package:flutter/foundation.dart';
 
 import 'box.dart';
+import 'object.dart';
 import 'sliver.dart';
 import 'sliver_fixed_extent_list.dart';
 import 'sliver_multi_box_adaptor.dart';
@@ -114,34 +115,84 @@ class RenderSliverFillRemaining extends RenderSliverSingleBoxAdapter {
   RenderSliverFillRemaining({
     RenderBox child,
     this.hasScrollBody = true,
+    this.fillOverscroll = false,
   }) : assert(hasScrollBody != null),
        super(child: child);
 
-  /// Whether the child has a scrollable body, this value cannot be null.
+  /// Indicates whether the child has a scrollable body, this value cannot be
+  /// null.
   ///
   /// Defaults to true such that the child will extend beyond the viewport and
   /// scroll, as seen in [NestedScrollView].
   ///
   /// Setting this value to false will allow the child to fill the remainder of
-  /// the viewport and not extend further.
+  /// the viewport and not extend further. However, if the
+  /// [precedingScrollExtent] exceeds the size of the viewport, the sliver will
+  /// defer to the child's size rather than overriding it.
   bool hasScrollBody;
 
+  /// Indicates whether the child should stretch to fill the overscroll area
+  /// created by certain scroll physics, such as iOS' default scroll physics.
+  /// This value cannot be null. This flag is only relevant when the
+  /// [hasScrollBody] value is false.
+  ///
+  /// Defaults to false, meaning the default behavior is for the child to
+  /// maintain its size and not extend into the overscroll area.
+  bool fillOverscroll;
+
   @override
   void performLayout() {
-    final double extent = constraints.remainingPaintExtent
-      - math.min(constraints.overlap, 0.0)
-      // Adding the offset for when this SliverFillRemaining is not scrollable,
-      // so it will stretch to fill on overscroll.
-      + (hasScrollBody ? 0.0 : constraints.scrollOffset);
-    if (child != null)
-      child.layout(constraints.asBoxConstraints(minExtent: extent, maxExtent: extent), parentUsesSize: true);
+    double childExtent;
+    double extent = constraints.viewportMainAxisExtent - constraints.precedingScrollExtent;
+    double maxExtent = constraints.remainingPaintExtent - math.min(constraints.overlap, 0.0);
+
+    if (hasScrollBody) {
+      extent = maxExtent;
+      if (child != null)
+        child.layout(
+          constraints.asBoxConstraints(
+            minExtent: extent,
+            maxExtent: extent,
+          ),
+          parentUsesSize: true,
+        );
+    } else if (child != null) {
+      child.layout(constraints.asBoxConstraints(), parentUsesSize: true);
+
+      switch (constraints.axis) {
+        case Axis.horizontal:
+          childExtent = child.size.width;
+          break;
+        case Axis.vertical:
+          childExtent = child.size.height;
+          break;
+      }
+      if (constraints.precedingScrollExtent > constraints.viewportMainAxisExtent || childExtent > extent)
+        extent = childExtent;
+      if (maxExtent < extent)
+        maxExtent = extent;
+      if ((fillOverscroll ? maxExtent : extent) > childExtent) {
+        child.layout(
+          constraints.asBoxConstraints(
+            minExtent: extent,
+            maxExtent: fillOverscroll ? maxExtent : extent,
+          ),
+          parentUsesSize: true,
+        );
+      }
+    }
+
+    assert(extent.isFinite,
+      'The calculated extent for the child of SliverFillRemaining is not finite.'
+        'This can happen if the child is a scrollable, in which case, the'
+        'hasScrollBody property of SliverFillRemaining should not be set to'
+        'false.',
+    );
     final double paintedChildSize = calculatePaintOffset(constraints, from: 0.0, to: extent);
     assert(paintedChildSize.isFinite);
     assert(paintedChildSize >= 0.0);
     geometry = SliverGeometry(
-      // 0.0 can be applied here for cases when there is not scroll body since
-      // SliverFillRemaining will not have any slivers following it.
-      scrollExtent: hasScrollBody ? constraints.viewportMainAxisExtent : 0.0,
+      scrollExtent: hasScrollBody ? constraints.viewportMainAxisExtent : extent,
       paintExtent: paintedChildSize,
       maxPaintExtent: paintedChildSize,
       hasVisualOverflow: extent > constraints.remainingPaintExtent || constraints.scrollOffset > 0.0,

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

@@ -1369,23 +1369,44 @@ class SliverFillRemaining extends SingleChildRenderObjectWidget {
     Key key,
     Widget child,
     this.hasScrollBody = true,
+    this.fillOverscroll = false,
   }) : assert(hasScrollBody != null),
        super(key: key, child: child);
 
-  /// Whether the child has a scrollable body, this value cannot be null.
+  /// Indicates whether the child has a scrollable body, this value cannot be
+  /// null.
   ///
   /// Defaults to true such that the child will extend beyond the viewport and
   /// scroll, as seen in [NestedScrollView].
   ///
   /// Setting this value to false will allow the child to fill the remainder of
-  /// the viewport and not extend further.
+  /// the viewport and not extend further. However, if the
+  /// [precedingScrollExtent] exceeds the size of the viewport, the sliver will
+  /// defer to the child's size rather than overriding it.
   final bool hasScrollBody;
 
+  /// Indicates whether the child should stretch to fill the overscroll area
+  /// created by certain scroll physics, such as iOS' default scroll physics.
+  /// This value cannot be null. This flag is only relevant when the
+  /// [hasScrollBody] value is false.
+  ///
+  /// Defaults to false, meaning the default behavior is for the child to
+  /// maintain its size and not extend into the overscroll area.
+  final bool fillOverscroll;
+
   @override
-  RenderSliverFillRemaining createRenderObject(BuildContext context) => RenderSliverFillRemaining(hasScrollBody: hasScrollBody);
+  RenderSliverFillRemaining createRenderObject(BuildContext context) {
+    return RenderSliverFillRemaining(
+      hasScrollBody: hasScrollBody,
+      fillOverscroll: fillOverscroll,
+    );
+  }
 
   @override
-  void updateRenderObject(BuildContext context, RenderSliverFillRemaining renderObject) => renderObject.hasScrollBody = hasScrollBody;
+  void updateRenderObject(BuildContext context, RenderSliverFillRemaining renderObject) {
+    renderObject.hasScrollBody = hasScrollBody;
+    renderObject.fillOverscroll = fillOverscroll;
+  }
 }
 
 /// Mark a child as needing to stay alive even when it's in a lazy list that