js-debug launches are session trees: a threadless root launcher spawns
child sessions (main script, [worker N]) via reverse startDebugging. Every
stateful command routed through a single #activeSessionId set on the last
registration or stop, so a worker attaching after the script stopped stole
focus. threads listed only the worker, post-launch breakpoints read back as
unbound, and step/continue/evaluate could not target the script thread.
Focus now follows stops, not registrations: a new session claims the active
pointer only when no live, stopped session already holds it. threads
aggregates every live thread across the tree, skipping the threadless
launcher while real children are alive.
Fixes#6663
roboomp
74c279c60b