The regression's script spun immediately after calling agent(), but the
agent() continuation (which posts the child-start RPC) only runs on a
microtask tick — the spin seized the worker's loop before the post, so
the host never saw a child inside the waitFor window. A few await-null
yields before the spin let the RPC out; the posted message needs no
further worker-loop turns to reach the host, so the wedge still holds
for the Cancel message the test is about.
imccyu
af9616f47d