As in Illustrator, the Pen shows its continue cursor over an end of any
open path, selected or not, and a click there selects the path and draws
on from that end (reversing it from its first end). While drawing, it
shows a join cursor over an end of another open path, and a click joins
the two into one path, in one undo step, which finishes the path.
path.join takes {ids: [a, b], ends: ["first"|"last", "first"|"last"]}
to join the ends asked for (the Pen's join); without them it joins the
selected paths' nearest ends as before. Joining a path with itself, or
anything that isn't a path, is an error.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>