updateChild method Null safety
Update the child render object if necessary.
Called before the first layout, any time markNeedsLayout is called, and
any time the scroll offset changes. The shrinkOffset is the difference
between the maxExtent and the current size. Zero means the header is
fully expanded, any greater number up to maxExtent means that the header
has been scrolled by that much. The overlapsContent argument is true if
the sliver's leading edge is beyond its normal place in the viewport
contents, and false otherwise. It may still paint beyond its normal place
if the minExtent after this call is greater than the amount of space that
would normally be left.
The render object will size itself to the larger of (a) the maxExtent minus the child's intrinsic height and (b) the maxExtent minus the shrink offset.
When this method is called by layoutChild, the child can be set, mutated, or replaced. (It should not be called outside layoutChild.)
Any time this method would mutate the child, call markNeedsLayout.
Implementation
@protected
void updateChild(double shrinkOffset, bool overlapsContent) { }