maxShowOnScreenExtent property Null safety
When a floating persistent header is told to show a Rect on screen, it may expand itself to accommodate the Rect. The maximum extent that is allowed for such expansion is either RenderSliverPersistentHeader.maxExtent or maxShowOnScreenExtent, whichever is smaller. If the persistent header's current extent is already larger than that maximum extent, it will remain unchanged.
This parameter can be set to the persistent header's
double.negativeInfinity) so the persistent header will never try to
expand when RenderObject.showOnScreen is called on it.
final double maxShowOnScreenExtent;