From e6ed069b326ab42b31c316b594277566d05b7bbc Mon Sep 17 00:00:00 2001
From: Michael Goderbauer <goderbauer@google.com>
Date: Wed, 6 Feb 2019 14:39:51 -0800
Subject: [PATCH] Fix initial scroll of TabBar in release mode (#27568)

---
 packages/flutter/lib/src/material/tabs.dart   | 13 +++-
 packages/flutter/test/material/tabs_test.dart | 61 +++++++++++++++++++
 2 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/packages/flutter/lib/src/material/tabs.dart b/packages/flutter/lib/src/material/tabs.dart
index a687324cc86..affb0a045d6 100644
--- a/packages/flutter/lib/src/material/tabs.dart
+++ b/packages/flutter/lib/src/material/tabs.dart
@@ -476,10 +476,21 @@ class _TabBarScrollPosition extends ScrollPositionWithSingleContext {
 
   final _TabBarState tabBar;
 
+  bool _initialViewportDimensionWasZero;
+
   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
     bool result = true;
-    if (pixels == null) {
+    if (_initialViewportDimensionWasZero != true) {
+      // If the viewport never had a non-zero dimension, we just want to jump
+      // to the initial scroll position to avoid strange scrolling effects in
+      // release mode: In release mode, the viewport temporarily may have a
+      // dimension of zero before the actual dimension is calculated. In that
+      // scenario, setting the actual dimension would cause a strange scroll
+      // effect without this guard because the super call below would starts a
+      // ballistic scroll activity.
+      assert(viewportDimension != null);
+      _initialViewportDimensionWasZero = viewportDimension != 0.0;
       correctPixels(tabBar._initialScrollOffset(viewportDimension, minScrollExtent, maxScrollExtent));
       result = false;
     }
diff --git a/packages/flutter/test/material/tabs_test.dart b/packages/flutter/test/material/tabs_test.dart
index 3d81d1b343b..02063e00606 100644
--- a/packages/flutter/test/material/tabs_test.dart
+++ b/packages/flutter/test/material/tabs_test.dart
@@ -2038,4 +2038,65 @@ void main() {
     expect(find.text(AlwaysKeepAliveWidget.text, skipOffstage: false), findsOneWidget);
     expect(find.text('4'), findsOneWidget);
   });
+
+  testWidgets('tabbar does not scroll when viewport dimensions initially change from zero to non-zero', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/10531.
+
+    const List<Widget> tabs = <Widget>[
+      Tab(text: 'NEW MEXICO'),
+      Tab(text: 'GABBA'),
+      Tab(text: 'HEY'),
+    ];
+    final TabController controller = TabController(vsync: const TestVSync(), length: tabs.length);
+
+    Widget buildTestWidget({double width, double height}) {
+      return MaterialApp(
+        home: Center(
+          child: SizedBox(
+            height: height,
+            width: width,
+            child: Scaffold(
+              appBar: AppBar(
+                title: const Text('AppBarBug'),
+                bottom: PreferredSize(
+                  preferredSize: const Size.fromHeight(30.0),
+                  child: Padding(
+                    padding: const EdgeInsets.symmetric(horizontal: 15.0),
+                    child: Align(
+                      alignment: FractionalOffset.center,
+                      child: TabBar(
+                        controller: controller,
+                        isScrollable: true,
+                        tabs: tabs,
+                      ),
+                    ),
+                  ),
+                ),
+              ),
+              body: const Center(
+                child: Text('Hello World'),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(
+      buildTestWidget(
+        width: 0.0,
+        height: 0.0,
+      ),
+    );
+
+    await tester.pumpWidget(
+      buildTestWidget(
+        width: 300.0,
+        height: 400.0,
+      ),
+    );
+
+    expect(tester.hasRunningAnimations, isFalse);
+    expect(await tester.pumpAndSettle(), 1); // no more frames are scheduled.
+  });
 }