flexibleSpace property
final
This widget is stacked behind the toolbar and the tabbar. It's height will be the same as the app bar's overall height.
A flexible space isn't actually flexible unless the AppBar's container changes the AppBar's size. A SliverAppBar in a CustomScrollView changes the AppBar's height when scrolled.
Typically a FlexibleSpaceBar. See FlexibleSpaceBar for details.
Implementation
final Widget flexibleSpace