sticky_header: Fix _findChildAtEnd when viewport partly consumed already

In particular this will affect the upper sliver (with older messages)
in the message list, when we start using two slivers there in earnest.

Author: gnprice

lib/widgets/sticky_header.dart

@@ -733,7 +733,10 @@ class _RenderSliverStickyHeaderListInner extends RenderSliverList {
   ///
   /// This means (child start) < (viewport end) <= (child end).
   RenderBox? _findChildAtEnd() {
-    final endOffset = constraints.scrollOffset + constraints.viewportMainAxisExtent;
+    /// The end of the visible area available to this sliver,
+    /// in this sliver's "scroll offset" coordinates.
+    final endOffset = constraints.scrollOffset
+      + constraints.remainingPaintExtent;
 
     RenderBox? child;
     for (child = lastChild; ; child = childBefore(child)) {

test/widgets/sticky_header_test.dart

@@ -153,6 +153,70 @@ void main() {
     // will get placed at zero rather than properly extend up off screen.
     check(tester.getTopLeft(find.text("Item 0"))).equals(Offset(0, -200));
   });
+
+  testWidgets('sliver only part of viewport, header at end', (tester) async {
+    const centerKey = ValueKey('center');
+    final controller = ScrollController();
+    await tester.pumpWidget(Directionality(textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        controller: controller,
+        anchor: 0.5,
+        center: centerKey,
+        slivers: [
+          SliverStickyHeaderList(
+            headerPlacement: HeaderPlacement.scrollingStart,
+            delegate: SliverChildListDelegate(
+              List.generate(100, (i) => StickyHeaderItem(
+                header: _Header(99 - i, height: 20),
+                child: _Item(99 - i, height: 100))))),
+          SliverStickyHeaderList(
+            key: centerKey,
+            headerPlacement: HeaderPlacement.scrollingStart,
+            delegate: SliverChildListDelegate(
+              List.generate(100, (i) => StickyHeaderItem(
+                header: _Header(100 + i, height: 20),
+                child: _Item(100 + i, height: 100))))),
+        ])));
+
+    final overallSize = tester.getSize(find.byType(CustomScrollView));
+    final extent = overallSize.onAxis(Axis.vertical);
+    assert(extent == 600);
+
+    void checkState(int index, {required double item, required double header}) {
+      final itemElement = tester.firstElement(find.byElementPredicate((element) {
+        if (element.widget is! _Item) return false;
+        final renderObject = element.renderObject as RenderBox;
+        return (renderObject.size.contains(renderObject.globalToLocal(
+            Offset(overallSize.width / 2, 1)
+        )));
+      }));
+      final itemWidget = itemElement.widget as _Item;
+      check(itemWidget.index).equals(index);
+      // TODO the `.first` calls should be unnecessary; that's another bug
+      // check(_headerIndex(tester)).equals(index);
+      check(tester.widget<_Header>(find.byType(_Header).first).index)
+        .equals(index);
+      check((itemElement.renderObject as RenderBox).localToGlobal(Offset(0, 0)))
+        .equals(Offset(0, item));
+      check(tester.getTopLeft(find.byType(_Header).first))
+        .equals(Offset(0, header));
+    }
+
+    check(controller.offset).equals(0);
+    checkState( 97, item:   0, header:   0);
+
+    controller.jumpTo(-5);
+    await tester.pump();
+    checkState( 96, item: -95, header: -15);
+
+    controller.jumpTo(-600);
+    await tester.pump();
+    checkState( 91, item:   0, header:   0);
+
+    controller.jumpTo(600);
+    await tester.pump();
+    checkState(103, item:   0, header:   0);
+  });
 }
 
 Future<void> _checkSequence(