Added expandedHeight in flexibleSpace and flexibleSpaceBar (#61710)

packages/flutter/lib/src/material/app_bar.dart

@@ -1123,6 +1123,9 @@ class SliverAppBar extends StatefulWidget {
   /// This widget is stacked behind the toolbar and the tab bar. It's height will
   /// be the same as the app bar's overall height.
   ///
+  /// When using [SliverAppBar.flexibleSpace], the [SliverAppBar.expandedHeight]
+  /// must be large enough to accommodate the [SliverAppBar.flexibleSpace] widget.
+  ///
   /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
   final Widget flexibleSpace;
 

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -41,10 +41,13 @@ enum StretchMode {
 /// The part of a material design [AppBar] that expands, collapses, and
 /// stretches.
 ///
-/// Most commonly used in in the [SliverAppBar.flexibleSpace] field, a flexible
+/// Most commonly used in the [SliverAppBar.flexibleSpace] field, a flexible
 /// space bar expands and contracts as the app scrolls so that the [AppBar]
 /// reaches from the top of the app to the top of the scrolling contents of the
-/// app. Furthermore is included functionality for stretch behavior. When
+/// app. When using [SliverAppBar.flexibleSpace], the [SliverAppBar.expandedHeight]
+/// must be large enough to accommodate the [SliverAppBar.flexibleSpace] widget.
+///
+/// Furthermore is included functionality for stretch behavior. When
 /// [SliverAppBar.stretch] is true, and your [ScrollPhysics] allow for
 /// overscroll, this space will stretch with the overscroll.
 ///