findDocumentSelection static method
Returns the current DocumentSelection for the SuperReader matched by
finder, or the singular SuperReader in the widget tree, if finder
is null.
By default, this method expects a single SuperReader in the widget tree and
finds it byType. To specify one SuperReader among many, pass a superDocumentFinder.
Implementation
static DocumentSelection? findDocumentSelection([Finder? finder]) {
  final element = (finder ?? find.byType(SuperReader)).evaluate().single as StatefulElement;
  final superDocument = element.state as SuperReaderState;
  return superDocument.selection;
}