Make scaffold support header and layout it on a custom layout

packages/remix/lib/src/components/scaffold/scaffold_widget.dart

@@ -1,5 +1,10 @@
 part of 'scaffold.dart';
 
+enum _ScaffoldElement {
+  header,
+  body;
+}
+
 /// A widget that provides a basic structure for a page or screen.
 ///
 /// It consists of a single [body] widget that is wrapped in a
@@ -26,11 +31,14 @@ part of 'scaffold.dart';
 class Scaffold extends StatelessWidget {
   const Scaffold({
     super.key,
+    this.header,
     required this.body,
     this.style,
     this.variants = const [],
   });
 
+  final Widget? header;
+
   /// The primary content of the scaffold.
   final Widget body;
 
@@ -50,8 +58,60 @@ class Scaffold extends StatelessWidget {
       builder: (context) {
         final spec = ScaffoldSpec.of(context);
 
-        return spec.container(child: ToastLayer(child: body));
+        return spec.container(
+          child: ToastLayer(
+            child: CustomMultiChildLayout(
+              delegate: _ScaffoldLayoutDelegate(),
+              children: [
+                if (header != null)
+                  LayoutId(
+                    id: _ScaffoldElement.header,
+                    child: header!,
+                  ),
+                LayoutId(
+                  id: _ScaffoldElement.body,
+                  child: body,
+                ),
+              ],
+            ),
+          ),
+        );
       },
     );
   }
 }
+
+class _ScaffoldLayoutDelegate extends MultiChildLayoutDelegate {
+  @override
+  void performLayout(Size size) {
+    final BoxConstraints looseConstraints = BoxConstraints.loose(size);
+    final BoxConstraints fullWidthConstraints =
+        looseConstraints.tighten(width: size.width);
+
+    double appBarHeight = 0.0;
+
+    const headerKey = _ScaffoldElement.header;
+    if (hasChild(headerKey)) {
+      appBarHeight = layoutChild(headerKey, fullWidthConstraints).height;
+      positionChild(headerKey, Offset.zero);
+    }
+
+    const bodyKey = _ScaffoldElement.body;
+    if (hasChild(bodyKey)) {
+      final listHeight = size.height - appBarHeight;
+
+      final bodyConstraints = BoxConstraints.tightFor(
+        width: fullWidthConstraints.maxWidth,
+        height: listHeight,
+      );
+
+      layoutChild(bodyKey, bodyConstraints);
+      positionChild(bodyKey, Offset(0, appBarHeight));
+    }
+  }
+
+  @override
+  bool shouldRelayout(covariant MultiChildLayoutDelegate oldDelegate) {
+    return false;
+  }
+}