sticky_header: Fix hit-testing of header

Our `hitTestChildren` implementation wasn't using `hitTestBoxChild`
correctly: it was attempting to do the job of `childMainAxisPosition`,
but `hitTestBoxChild` calls `childMainAxisPosition` for itself.

The implementation of `childMainAxisPosition` wasn't correct either.

Fix both, and add some tests; sticky_header hadn't had any tests on
its hit-test behavior.

Fixes: #327

lib/widgets/sticky_header.dart

@@ -592,11 +592,8 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
     assert(child != null);
     assert(geometry!.hitTestExtent > 0.0);
     if (header != null) {
-      final headerParentData = (header!.parentData as SliverPhysicalParentData);
-      final headerOffset = headerParentData.paintOffset
-        .inDirection(constraints.axisDirection);
       if (hitTestBoxChild(BoxHitTestResult.wrap(result), header!,
-            mainAxisPosition: mainAxisPosition - headerOffset,
+            mainAxisPosition: mainAxisPosition,
             crossAxisPosition: crossAxisPosition)) {
         return true;
       }
@@ -609,8 +606,17 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
   double childMainAxisPosition(RenderObject child) {
     if (child == this.child) return 0.0;
     assert(child == header);
+    // We use Sliver*Physical*ParentData, so the header's position is stored in
+    // physical coordinates.  To meet the spec of `childMainAxisPosition`, we
+    // need to convert to the sliver's coordinate system.
     final headerParentData = (header!.parentData as SliverPhysicalParentData);
-    return headerParentData.paintOffset.inDirection(constraints.axisDirection);
+    final paintOffset = headerParentData.paintOffset;
+    return switch (constraints.axisDirection) {
+      AxisDirection.right => paintOffset.dx,
+      AxisDirection.left  => geometry!.layoutExtent - header!.size.width  - paintOffset.dx,
+      AxisDirection.down  => paintOffset.dy,
+      AxisDirection.up    => geometry!.layoutExtent - header!.size.height - paintOffset.dy,
+    };
   }
 
   @override

test/widgets/sticky_header_test.dart

@@ -132,9 +132,17 @@ Future<void> _checkSequence(
         header: _Header(i, height: 20),
         child: _Item(i, height: 100))))));
 
-  final extent = tester.getSize(find.byType(StickyHeaderListView)).onAxis(axis);
+  final overallSize = tester.getSize(find.byType(StickyHeaderListView));
+  final extent = overallSize.onAxis(axis);
   assert(extent % 100 == 0);
 
+  // A position `inset` from the center of the edge the header is found on.
+  Offset headerInset(double inset) {
+    return overallSize.center(Offset.zero)
+      + offsetInDirection(axis.coordinateDirection,
+          (extent / 2 - inset) * (headerAtCoordinateEnd ? 1 : -1));
+  }
+
   final first = !(reverse ^ reverseHeader);
 
   final itemFinder = first ? find.byType(_Item).first : find.byType(_Item).last;
@@ -145,28 +153,39 @@ Future<void> _checkSequence(
       : tester.getBottomRight(finder).inDirection(axis.coordinateDirection);
   }
 
-  void checkState() {
+  Future<void> checkState() async {
+    // Check the header comes from the expected item.
     final scrollOffset = controller.position.pixels;
     final expectedHeaderIndex = first
       ? (scrollOffset / 100).floor()
       : (extent ~/ 100 - 1) + (scrollOffset / 100).ceil();
     check(tester.widget<_Item>(itemFinder).index).equals(expectedHeaderIndex);
     check(_headerIndex(tester)).equals(expectedHeaderIndex);
 
+    // Check the layout of the header and item.
     final expectedItemInsetExtent =
       100 - (first ? scrollOffset % 100 : (-scrollOffset) % 100);
+    final double expectedHeaderInsetExtent =
+      allowOverflow ? 20 : math.min(20, expectedItemInsetExtent);
     check(insetExtent(itemFinder)).equals(expectedItemInsetExtent);
-    check(insetExtent(find.byType(_Header))).equals(
-      allowOverflow ? 20 : math.min(20, expectedItemInsetExtent));
+    check(insetExtent(find.byType(_Header))).equals(expectedHeaderInsetExtent);
+
+    // Check the header gets hit when it should, and not when it shouldn't.
+    await tester.tapAt(headerInset(1));
+    await tester.tapAt(headerInset(expectedHeaderInsetExtent - 1));
+    check(_Header.takeTapCount()).equals(2);
+    await tester.tapAt(headerInset(extent - 1));
+    await tester.tapAt(headerInset(extent - (expectedHeaderInsetExtent - 1)));
+    check(_Header.takeTapCount()).equals(0);
   }
 
   Future<void> jumpAndCheck(double position) async {
     controller.jumpTo(position);
     await tester.pump();
-    checkState();
+    await checkState();
   }
 
-  checkState();
+  await checkState();
   await jumpAndCheck(5);
   await jumpAndCheck(10);
   await jumpAndCheck(20);
@@ -210,12 +229,21 @@ class _Header extends StatelessWidget {
   final int index;
   final double height;
 
+  static int takeTapCount() {
+    final result = _tapCount;
+    _tapCount = 0;
+    return result;
+  }
+  static int _tapCount = 0;
+
   @override
   Widget build(BuildContext context) {
     return SizedBox(
       height: height,
       width: height, // TODO clean up
-      child: Text("Header $index"));
+      child: GestureDetector(
+        onTap: () => _tapCount++,
+        child: Text("Header $index")));
   }
 }