[scrollable_positioned_list] Fix default cache extents for horizontal (#463)

* [scrollable_positioned_list] Fix default cache extents for horizontal lists.

Closes #439 #453

* Remove unneeded import

* Remove unneeded unwrap

Author: tarobins

packages/scrollable_positioned_list/lib/src/scrollable_positioned_list.dart

@@ -275,7 +275,7 @@ class _ScrollablePositionedListState extends State<ScrollablePositionedList>
   @override
   void initState() {
     super.initState();
-    ItemPosition? initialPosition = PageStorage.of(context)!.readState(context);
+    ItemPosition? initialPosition = PageStorage.of(context).readState(context);
     primary.target = initialPosition?.index ?? widget.initialScrollIndex;
     primary.alignment =
         initialPosition?.itemLeadingEdge ?? widget.initialAlignment;
@@ -410,7 +410,10 @@ class _ScrollablePositionedListState extends State<ScrollablePositionedList>
   }
 
   double _cacheExtent(BoxConstraints constraints) => max(
-        constraints.maxHeight * _screenScrollCount,
+        (widget.scrollDirection == Axis.vertical
+                ? constraints.maxHeight
+                : constraints.maxWidth) *
+            _screenScrollCount,
         widget.minCacheExtent ?? 0,
       );
 
@@ -568,7 +571,7 @@ class _ScrollablePositionedListState extends State<ScrollablePositionedList>
         .where((ItemPosition position) =>
             position.itemLeadingEdge < 1 && position.itemTrailingEdge > 0);
     if (itemPositions.isNotEmpty) {
-      PageStorage.of(context)!.writeState(
+      PageStorage.of(context).writeState(
           context,
           itemPositions.reduce((value, element) =>
               value.itemLeadingEdge < element.itemLeadingEdge

packages/scrollable_positioned_list/test/horizontal_scrollable_positioned_list_test.dart

@@ -7,7 +7,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:pedantic/pedantic.dart';
 import 'package:scrollable_positioned_list/scrollable_positioned_list.dart';
 
-const screenHeight = 400.0;
+const screenHeight = 100.0;
 const screenWidth = 400.0;
 const itemWidth = screenWidth / 10.0;
 const itemCount = 500;
@@ -45,6 +45,11 @@ void main() {
     );
   }
 
+  final Finder fadeTransitionFinder = find.descendant(
+    of: find.byType(ScrollablePositionedList),
+    matching: find.byType(FadeTransition),
+  );
+
   testWidgets('List positioned with 0 at left', (WidgetTester tester) async {
     final itemPositionsListener = ItemPositionsListener.create();
     await setUpWidgetTest(tester, itemPositionsListener: itemPositionsListener);
@@ -171,7 +176,7 @@ void main() {
     await tester.pumpAndSettle();
 
     expect(tester.getTopLeft(find.text('Item 100')).dx, 0);
-    expect(tester.getBottomRight(find.text('Item 109')).dy, screenWidth);
+    expect(tester.getBottomRight(find.text('Item 109')).dy, screenHeight);
 
     expect(
         itemPositionsListener.itemPositions.value
@@ -195,6 +200,31 @@ void main() {
         1);
   });
 
+  testWidgets('Scroll to 20 without fading', (WidgetTester tester) async {
+    final itemScrollController = ItemScrollController();
+    final itemPositionsListener = ItemPositionsListener.create();
+    await setUpWidgetTest(tester,
+        itemScrollController: itemScrollController,
+        itemPositionsListener: itemPositionsListener);
+
+    var fadeTransition = tester.widget<FadeTransition>(fadeTransitionFinder);
+    final initialOpacity = fadeTransition.opacity;
+
+    unawaited(
+        itemScrollController.scrollTo(index: 20, duration: scrollDuration));
+    await tester.pump();
+    await tester.pump();
+    await tester.pump(scrollDuration ~/ 2);
+
+    fadeTransition = tester.widget<FadeTransition>(fadeTransitionFinder);
+    expect(fadeTransition.opacity, initialOpacity);
+
+    await tester.pumpAndSettle();
+
+    expect(find.text('Item 14'), findsNothing);
+    expect(find.text('Item 20'), findsOneWidget);
+  });
+
   testWidgets('padding test - centered sliver at left',
       (WidgetTester tester) async {
     final itemScrollController = ItemScrollController();