If true, and an appBar is specified, then the height of the body is extended to include the height of the app bar and the top of the body is aligned with the top of the app bar.
This is useful if the app bar's AppBar.backgroundColor is not completely opaque.
This property is false by default. It must not be null.
- extendBody, which extends the height of the body to the bottom of the scaffold.
final bool extendBodyBehindAppBar;