sticky_header [nfc]: Fix childMainAxisPosition to handle paintOrigin nonzero

This fixes a latent bug: this method would give wrong answers if the
sliver's paintOrigin were nonzero.  See the new comments.

The bug is latent because performLayout currently always produces a
zero paintOrigin.  But we'll start using paintOrigin soon, as part of
making hit-testing work correctly when a sticky header is painted by
one sliver but needs to encroach on the layout area of another sliver.
The framework calls this method as part of hit-testing, so that
requires fixing this bug too.

Author: gnprice

lib/widgets/sticky_header.dart

@@ -691,17 +691,28 @@ 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.
+
+    // First, use the sliver's layout coordinates (where the origin is the
+    // point where the sliver's layout began, corresponding to
+    // `constraints.scrollOffset`):
     final headerParentData = (header!.parentData as SliverPhysicalParentData);
     final paintOffset = headerParentData.paintOffset;
-    return switch (constraints.growthAxisDirection) {
+    final fromScrollOffset = switch (constraints.growthAxisDirection) {
       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,
     };
+
+    // Then the [childMainAxisPosition] spec wants a location relative to the
+    // "leading _visible_ edge" of this sliver; and "visible" means paint,
+    // not layout.  The leading visible edge, expressed again in the sliver's
+    // layout coordinates, is at `paintOrigin`.
+    return fromScrollOffset - geometry!.paintOrigin;
   }
 
   @override