minExtent property
The smallest size to allow the header to reach, when it shrinks at the start of the viewport.
This must return a value equal to or less than maxExtent.
This value should not change over the lifetime of the delegate. It should be based entirely on the constructor arguments passed to the delegate. See shouldRebuild, which must return true if a new delegate would return a different value.
Implementation
@override
double get minExtent => horizontal ? width : height;