extentOf method
Returns the best-known full (non-animated) extent for key: the
measured value if the node has ever been laid out, otherwise
extentEstimator if supplied, otherwise defaultExtent. Matches the
fallback chain used by scrollOffsetOf.
Implementation
double extentOf(TKey key, {double Function(TKey key)? extentEstimator}) =>
_scroll.extentOf(key, extentEstimator: extentEstimator);