config

This browser's own copy of the configuration it was created with.

start resolves runtime state onto it — it assigns Config.host / Config.port and appends launch arguments — so it must not be the caller's instance. Mutating a shared config would make any later browser built from it take the connectExisting branch of start: it would skip launching a browser process entirely and try to reach the previous port, which is dead. Copying here makes reusing (and retrying with) the same Config safe by construction.