Flutter Sliver Sticky Header hide content behind rounded corners

I implemented a SliverStickyHeader which is working just fine within my CustomScrollView. I only have one problem:

My Sticky header is basically a Container with borderRadius. Now I want my content to hide behind that sliver so only the Scaffold backgorund is visible. Here is an image for clarification:

As you can see the white Container is visible behind the blue SliverStickyHeader.

I am stuck here and have no clue if this is even possible. Happy for every help, even if it's hacky :D Would love to make it work.

Hiding the Content of my Sliver behind the Header. That's the goal.

Let me know if you need any more information.

Adding it inside the delegate is working for me:

class WishlistSliverPersistentHeaderDelegate
    extends SliverPersistentHeaderDelegate {
  ...

  WishlistSliverPersistentHeaderDelegate({
    ...
  });

  @override
  Widget build(
    BuildContext context,
    double shrinkOffset,
    bool overlapsContent,
  ) {
    final t = shrinkOffset / (maxExtent);
    final collapsedOffset = ui.lerpDouble(0, 1, t)!;
    final collapsedOffsetSafe = collapsedOffset.safeOpacity;
    return Column(
      mainAxisAlignment: MainAxisAlignment.end,
      children: [
        Expanded(
          child: Stack(
            alignment: Alignment.topCenter,
            children: [
              _buildHeader(context, t),
              ...
              _builderBorderRadius(context),
            ],
          ),
        ),
      ],
    );
  }

  Positioned _builderBorderRadius(BuildContext context) {
    return Positioned(
      bottom: 0,
      left: 0,
      right: 0,
      child: Container(
        height: 30,
        decoration: BoxDecoration(
          color: whiteAdaptive(context),
          borderRadius: const BorderRadius.only(
            topLeft: Radius.circular(30),
            topRight: Radius.circular(30),
          ),
        ),
      ),
    );
  }