The number of children that will contribute semantic information.
Some subtypes of ScrollView can infer this value automatically. For example ListView will use the number of widgets in the child list, while the new ListView.separated constructor will use half that amount.
For CustomScrollView and other types which do not receive a builder or list of widgets, the child count must be explicitly provided. If the number is unknown or unbounded this should be left unset or set to null.
- SemanticsConfiguration.scrollChildCount, the corresponding semantics property.
final int semanticChildCount