sticky_header test [nfc]: Make "first/last item" finders more robust

The existing `.first` and `.last` versions rely on the order that
children appear in the Flutter element tree.

As is, that works because the `_Item` widgets are all children of one
list sliver, and it manages its children in a nice straightforward
order.  But when we add multiple list slivers, the order will depend
also on the order those appear as children of the viewport; and in
particular the items at the edges of the viewport will no longer
always be the first or last items in the tree.  So introduce a couple
of custom finders to keep finding the first or last items in the
sense we mean.

For examples of using the APIs these finders use, compare the
implementation of [FinderBase.first].

Author: gnprice

test/widgets/sticky_header_test.dart

@@ -277,9 +277,10 @@ Future<void> _checkSequence(
 
   final first = !(reverse ^ reverseHeader ^ reverseGrowth);
 
-  final itemFinder = first ? find.byType(_Item).first : find.byType(_Item).last;
+  final itemFinder = first ? _LeastItemFinder(find.byType(_Item))
+                           : _GreatestItemFinder(find.byType(_Item));
 
-  double insetExtent(Finder finder) {
+  double insetExtent(FinderBase<Element> finder) {
     return headerAtCoordinateEnd
       ? extent - tester.getTopLeft(finder).inDirection(axis.coordinateDirection)
       : tester.getBottomRight(finder).inDirection(axis.coordinateDirection);
@@ -330,6 +331,64 @@ Future<void> _checkSequence(
   await jumpAndCheck(100);
 }
 
+abstract class _SelectItemFinder extends FinderBase<Element> with ChainedFinderMixin<Element> {
+  bool shouldPrefer(_Item candidate, _Item previous);
+
+  @override
+  Iterable<Element> filter(Iterable<Element> parentCandidates) {
+    Element? result;
+    _Item? resultWidget;
+    for (final candidate in parentCandidates) {
+      if (candidate is! ComponentElement) continue;
+      final widget = candidate.widget;
+      if (widget is! _Item) continue;
+      if (resultWidget == null || shouldPrefer(widget, resultWidget)) {
+        result = candidate;
+        resultWidget = widget;
+      }
+    }
+    return [if (result != null) result];
+  }
+}
+
+/// Finds the [_Item] with least [_Item.index]
+/// out of all elements found by the given parent finder.
+class _LeastItemFinder extends _SelectItemFinder {
+  _LeastItemFinder(this.parent);
+
+  @override
+  final FinderBase<Element> parent;
+
+  @override
+  String describeMatch(Plurality plurality) {
+    return 'least-index _Item from ${parent.describeMatch(plurality)}';
+  }
+
+  @override
+  bool shouldPrefer(_Item candidate, _Item previous) {
+    return candidate.index < previous.index;
+  }
+}
+
+/// Finds the [_Item] with greatest [_Item.index]
+/// out of all elements found by the given parent finder.
+class _GreatestItemFinder extends _SelectItemFinder {
+  _GreatestItemFinder(this.parent);
+
+  @override
+  final FinderBase<Element> parent;
+
+  @override
+  String describeMatch(Plurality plurality) {
+    return 'greatest-index _Item from ${parent.describeMatch(plurality)}';
+  }
+
+  @override
+  bool shouldPrefer(_Item candidate, _Item previous) {
+    return candidate.index > previous.index;
+  }
+}
+
 Future<void> _drag(WidgetTester tester, Offset offset) async {
   await tester.drag(find.byType(StickyHeaderListView), offset);
   await tester.pump();