If non-null, determines the default behavior for each method.
If a subclass of ScrollPhysics does not override a method, that subclass will inherit an implementation from this base class that defers to parent. This mechanism lets you assemble novel combinations of ScrollPhysics subclasses at runtime. For example:
const BouncingScrollPhysics(parent: AlwaysScrollableScrollPhysics())
will result in a ScrollPhysics that has the combined behavior of BouncingScrollPhysics and AlwaysScrollableScrollPhysics: behaviors that are not specified in BouncingScrollPhysics (e.g. shouldAcceptUserOffset) will defer to AlwaysScrollableScrollPhysics.
final ScrollPhysics? parent;