flexibleSpace property
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.
A flexible space isn't actually flexible unless the GFAppBar's container changes the GFAppBar's size. A SliverGFAppBar in a CustomScrollView changes the GFAppBar's height when scrolled.
Typically a FlexibleSpaceBar. See FlexibleSpaceBar for details.
Implementation
final Widget? flexibleSpace;