trackVisibility property Null safety
Indicates that the scrollbar track should be visible.
When true, the scrollbar track will always be visible so long as the thumb is visible. If the scrollbar thumb is not visible, the track will not be visible either.
Defaults to false when null.
Subclass Scrollbar can hide and show the scrollbar thumb in response to MaterialStates by using ScrollbarThemeData.trackVisibility.
final bool? trackVisibility;