Revert "Fix the maths for floating pinned app bars. (#11996)" (#12071)

This reverts commit da0602fe.

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

@@ -708,6 +708,7 @@ 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.minExtent;
+    final double minExtent = this.maxExtent;
     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

-// 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));
-  });
-}
\ No newline at end of file