PageView ballistics overshoot the page on some devices (#12884)

* PageView ballistics overshoot the page on some devices

On some devices, such as Cupertino “Plus”-sized devices, scrolling left on the first page of a PageView will overshoot the first page and land on the second page.

The issue is that applyContentDimensions incorrectly detects a content size change due to a floating point comparison on certain screen sizes (18257.400000000005 vs 18257.4)

To fix this, perform a nearEqual comparison in applyContentDimensions.

* Apply style changes to nearEqual for code review feedback.

packages/flutter/lib/src/physics/utils.dart

@@ -4,9 +4,14 @@
 
 /// Whether two doubles are within a given distance of each other.
 ///
-/// The epsilon argument must be positive.
+/// The `epsilon` argument must be positive and not null.
+/// The `a` and `b` arguments may be null. A null value is only considered
+/// near-equal to another null value.
 bool nearEqual(double a, double b, double epsilon) {
+  assert(epsilon != null);
   assert(epsilon >= 0.0);
+  if (a == null || b == null)
+    return a == b;
   return (a > (b - epsilon)) && (a < (b + epsilon));
 }
 

packages/flutter/lib/src/widgets/scroll_position.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
+import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
@@ -412,8 +413,8 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
 
   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
-    if (_minScrollExtent != minScrollExtent ||
-        _maxScrollExtent != maxScrollExtent ||
+    if (!nearEqual(_minScrollExtent, minScrollExtent, Tolerance.defaultTolerance.distance) ||
+        !nearEqual(_maxScrollExtent, maxScrollExtent, Tolerance.defaultTolerance.distance) ||
         _didChangeViewportDimension) {
       _minScrollExtent = minScrollExtent;
       _maxScrollExtent = maxScrollExtent;

packages/flutter/test/physics/near_equal_test.dart

@@ -13,4 +13,10 @@ void main() {
     expect(nearEqual(5.0, 6.0, 0.5), isFalse);
     expect(nearEqual(6.0, 5.0, 0.5), isFalse);
   });
+
+  test('test_null', () {
+    expect(nearEqual(5.0, null, 2.0), isFalse);
+    expect(nearEqual(null, 5.0, 2.0), isFalse);
+    expect(nearEqual(null, null, 2.0), isTrue);
+  });
 }

packages/flutter/test/widgets/page_view_test.dart

@@ -325,6 +325,60 @@ void main() {
     expect(find.text('Alaska'), findsOneWidget);
   });
 
+  testWidgets('Bouncing scroll physics ballistics does not overshoot', (WidgetTester tester) async {
+    final List<int> log = <int>[];
+    final PageController controller = new PageController(viewportFraction: 0.9);
+
+    Widget build(PageController controller, {Size size}) {
+      final Widget pageView = new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new PageView(
+          controller: controller,
+          onPageChanged: log.add,
+          physics: const BouncingScrollPhysics(),
+          children: kStates.map<Widget>((String state) => new Text(state)).toList(),
+        ),
+      );
+
+      if (size != null) {
+        return new OverflowBox(
+          child: pageView,
+          minWidth: size.width,
+          minHeight: size.height,
+          maxWidth: size.width,
+          maxHeight: size.height,
+        );
+      } else {
+        return pageView;
+      }
+    }
+
+    await tester.pumpWidget(build(controller));
+    expect(log, isEmpty);
+
+    // Fling right to move to a non-existent page at the beginning of the
+    // PageView, and confirm that the PageView settles back on the first page.
+    await tester.fling(find.byType(PageView), const Offset(100.0, 0.0), 800.0);
+    await tester.pumpAndSettle();
+    expect(log, isEmpty);
+
+    expect(find.text('Alabama'), findsOneWidget);
+    expect(find.text('Alaska'), findsOneWidget);
+    expect(find.text('Arizona'), findsNothing);
+
+    // Try again with a Cupertino "Plus" device size.
+    await tester.pumpWidget(build(controller, size: const Size(414.0, 736.0)));
+    expect(log, isEmpty);
+
+    await tester.fling(find.byType(PageView), const Offset(100.0, 0.0), 800.0);
+    await tester.pumpAndSettle();
+    expect(log, isEmpty);
+
+    expect(find.text('Alabama'), findsOneWidget);
+    expect(find.text('Alaska'), findsOneWidget);
+    expect(find.text('Arizona'), findsNothing);
+  });
+
   testWidgets('PageView viewportFraction', (WidgetTester tester) async {
     PageController controller = new PageController(viewportFraction: 7/8);