Don't crash if pinned & floating AppBar has less than minExtent remainingPaintExtent (#25048)

Fixes https://github.com/flutter/flutter/issues/21887

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

@@ -515,13 +515,15 @@ abstract class RenderSliverFloatingPinnedPersistentHeader extends RenderSliverFl
   @override
   double updateGeometry() {
     final double minExtent = this.minExtent;
+    final double minAllowedExtent = constraints.remainingPaintExtent > minExtent ? minExtent : constraints.remainingPaintExtent;
     final double maxExtent = this.maxExtent;
     final double paintExtent = maxExtent - _effectiveScrollOffset;
+    final double clampedPaintExtent = paintExtent.clamp(minAllowedExtent, constraints.remainingPaintExtent);
     final double layoutExtent = maxExtent - constraints.scrollOffset;
     geometry = SliverGeometry(
       scrollExtent: maxExtent,
-      paintExtent: paintExtent.clamp(minExtent, constraints.remainingPaintExtent),
-      layoutExtent: layoutExtent.clamp(0.0, constraints.remainingPaintExtent - minExtent),
+      paintExtent: clampedPaintExtent,
+      layoutExtent: layoutExtent.clamp(0.0, clampedPaintExtent),
       maxPaintExtent: maxExtent,
       maxScrollObstructionExtent: maxExtent,
       hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.

packages/flutter/test/widgets/slivers_appbar_floating_pinned_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/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 
@@ -66,4 +67,49 @@ void main() {
     // we've scrolled that up by 600.0, meaning it's at that minus 600 now:
     expect(tester.getTopLeft(find.text('E')), Offset(0.0, 200.0 + 56.0 + cSize.height * 2.0 + 500.0 - 600.0));
   });
+
+  testWidgets('Does not crash when there is less than minExtent remainingPaintExtent', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/21887.
+    final ScrollController controller = ScrollController();
+    const double availableHeight = 50.0;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Center(
+          child: Container(
+            height: availableHeight,
+            color: Colors.green,
+            child: CustomScrollView(
+              controller: controller,
+              slivers: <Widget>[
+                const SliverAppBar(
+                  pinned: true,
+                  floating: true,
+                  expandedHeight: 120.0,
+                ),
+                SliverList(
+                  delegate: SliverChildListDelegate(List<Widget>.generate(20, (int i) {
+                    return Container(
+                      child: Text('Tile $i'),
+                      height: 100.0,
+                    );
+                  })),
+                )
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+    final RenderSliverFloatingPinnedPersistentHeader render = tester.renderObject(find.byType(SliverAppBar));
+    expect(render.minExtent, greaterThan(availableHeight)); // Precondition
+    expect(render.geometry.scrollExtent, 120.0);
+    expect(render.geometry.paintExtent, availableHeight);
+    expect(render.geometry.layoutExtent, availableHeight);
+
+    controller.jumpTo(200.0);
+    await tester.pumpAndSettle();
+    expect(render.geometry.scrollExtent, 120.0);
+    expect(render.geometry.paintExtent, availableHeight);
+    expect(render.geometry.layoutExtent, 0.0);
+  });
 }