primary property Null safety
Whether this is the primary scroll view associated with the parent PrimaryScrollController.
When true, the scroll view is used for default ScrollActions. If a ScrollAction is not handled by an otherwise focused part of the application, the ScrollAction will be evaluated using this scroll view, for example, when executing Shortcuts key events like page up and down.
On iOS, this identifies the scroll view that will scroll to top in response to a tap in the status bar.
final bool primary;