minShowOnScreenExtent 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 minimum extent that is allowed for such expansion is either RenderSliverPersistentHeader.minExtent or minShowOnScreenExtent, whichever is larger. 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.infinity) so the persistent header will always try to expand when
RenderObject.showOnScreen is called on it.
final double minShowOnScreenExtent;