startSelection method
Starts a selection (tap or start of drag)
Implementation
void startSelection(String nodeId, String fragmentId, int offset) {
final point = SelectionPoint(
nodeId: nodeId,
fragmentId: fragmentId,
offset: offset,
);
_state = SelectionState(anchor: point, focus: point);
_rangeCache.clear();
if (!_suppressNotifications) notifyListeners();
}