Fixed issue for SliverAppBar collapsedHeight (#72300)

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

@@ -1464,7 +1464,7 @@ class SliverAppBar extends StatefulWidget {
        assert(toolbarHeight != null),
        assert(floating || !snap, 'The "snap" argument only makes sense for floating app bars.'),
        assert(stretchTriggerOffset > 0.0),
-       assert(collapsedHeight == null || collapsedHeight > toolbarHeight, 'The "collapsedHeight" argument has to be larger than [toolbarHeight].'),
+       assert(collapsedHeight == null || collapsedHeight >= toolbarHeight, 'The "collapsedHeight" argument has to be larger than or equal to [toolbarHeight].'),
        super(key: key);
 
   /// {@macro flutter.material.appbar.leading}

packages/flutter/test/material/app_bar_test.dart

@@ -2198,6 +2198,25 @@ void main() {
     expect(appBarHeight(tester), collapsedHeight + initialTabBarHeight);
   });
 
+  testWidgets('SliverAppBar collapsedHeight', (WidgetTester tester) async {
+    const double collapsedHeight = 56.0;
+
+    await tester.pumpWidget(buildSliverAppBarApp(
+        floating: false,
+        pinned: false,
+        collapsedHeight: collapsedHeight
+    ));
+
+    final ScrollController controller = primaryScrollController(tester);
+    final double initialTabBarHeight = tabBarHeight(tester);
+
+    // Scroll the not-pinned appbar out of view, to its collapsed height.
+    controller.jumpTo(300.0);
+    await tester.pump();
+    expect(find.byType(SliverAppBar), findsNothing);
+    expect(appBarHeight(tester), collapsedHeight + initialTabBarHeight);
+  });
+
   testWidgets('AppBar respects leadingWidth', (WidgetTester tester) async {
     const Key key = Key('leading');
     await tester.pumpWidget(MaterialApp(