To complete the polyline as an open vector, and remain in polyline creation mode:
To complete the polyline as a closed vector, by joining its first and last nodes with a linear span, and remain in polyline creation mode:
To complete the polyline as an open vector and close the Tool Settings: Polyline Creation panel:
A completed polyline is displayed in magenta and is surrounded by a bounding box, indicating that it is an ungrouped vector.