I think there is a scrolling conflict between the CustomScrollView and the WebView. To resolve this, you can disable scrolling in the CustomScrollView by using NeverScrollableScrollPhysics.
@override
Widget build(BuildContext context) {
return CustomScrollView(
physics: NeverScrollableScrollPhysics(), // Disables scrolling in CustomScrollView
slivers: <Widget>[
SliverAppBar(
title: const Text("Heading"),
floating: true,
),
SliverFillRemaining(
child: WebView(initialUrl: "http://stackoverflow.com"),
),
],
);
}