Fix the maths for floating pinned app bars. (#11996)

packages/flutter/lib/src/material/app_bar.dart

@@ -708,7 +708,6 @@ class SliverAppBar extends StatefulWidget {
        assert(primary != null),
        assert(floating != null),
        assert(pinned != null),
-       assert(!pinned || !floating || bottom != null, 'A pinned and floating app bar must have a bottom widget.'),
        assert(snap != null),
        assert(floating || !snap, 'The "snap" argument only makes sense for floating app bars.'),
        super(key: key);

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

@@ -512,7 +512,7 @@ abstract class RenderSliverFloatingPinnedPersistentHeader extends RenderSliverFl
 
   @override
   double updateGeometry() {
-    final double minExtent = this.maxExtent;
+    final double minExtent = this.minExtent;
     final double maxExtent = this.maxExtent;
     final double paintExtent = (maxExtent - _effectiveScrollOffset);
     final double layoutExtent = (maxExtent - constraints.scrollOffset);

packages/flutter/test/widgets/slivers_appbar_floating_pinned_test.dart

@@ -0,0 +1,62 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+
+void main() {
+  testWidgets('Sliver appbars - floating and pinned - second app bar stacks below', (WidgetTester tester) async {
+    final ScrollController controller = new ScrollController();
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new MediaQuery(
+          data: const MediaQueryData(),
+          child: new CustomScrollView(
+            controller: controller,
+            slivers: <Widget>[
+              const SliverAppBar(floating: true, pinned: true, expandedHeight: 200.0, title: const Text('A')),
+              const SliverAppBar(primary: false, pinned: true, title: const Text('B')),
+              new SliverList(
+                delegate: new SliverChildListDelegate(
+                  <Widget>[
+                    const Text('C'),
+                    const Text('D'),
+                    const SizedBox(height: 500.0),
+                    const Text('E'),
+                    const SizedBox(height: 500.0),
+                  ],
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+    const Offset textPositionInAppBar = const Offset(16.0, 18.0);
+    expect(tester.getTopLeft(find.text('A')), textPositionInAppBar);
+    // top app bar is 200.0 high at this point
+    expect(tester.getTopLeft(find.text('B')), const Offset(0.0, 200.0) + textPositionInAppBar);
+    // second app bar is 56.0 high
+    expect(tester.getTopLeft(find.text('C')), const Offset(0.0, 200.0 + 56.0)); // height of both appbars
+    final Size cSize = tester.getSize(find.text('C'));
+    controller.jumpTo(200.0 - 56.0);
+    await tester.pump();
+    expect(tester.getTopLeft(find.text('A')), textPositionInAppBar);
+    // top app bar is now only 56.0 high, same as second
+    expect(tester.getTopLeft(find.text('B')), const Offset(0.0, 56.0) + textPositionInAppBar);
+    expect(tester.getTopLeft(find.text('C')), const Offset(0.0, 56.0 * 2.0)); // height of both collapsed appbars
+    expect(find.text('E'), findsNothing);
+    controller.jumpTo(600.0);
+    await tester.pump();
+    expect(tester.getTopLeft(find.text('A')), textPositionInAppBar); // app bar is pinned at top
+    expect(tester.getTopLeft(find.text('B')), const Offset(0.0, 56.0) + textPositionInAppBar); // second one too
+    expect(find.text('C'), findsNothing); // contents are scrolled off though
+    expect(find.text('D'), findsNothing);
+    // we have scrolled 600.0 pixels
+    // initial position of E was 200 + 56 + cSize.height + cSize.height + 500
+    // we've scrolled that up by 600.0, meaning it's at that minus 600 now:
+    expect(tester.getTopLeft(find.text('E')), new Offset(0.0, 200.0 + 56.0 + cSize.height * 2.0 + 500.0 - 600.0));
+  });
+}