Auto-completion after `db.` sends a listCollection to the server, which may be blocked on locks. In 4.0, the lifespan of a lock could be much longer than before due to transactions. When a 'dropDatabase' is waiting on the global lock in X mode which is acquired by an inactive transaction in IX mode, listCollection will be blocked. The behavior is expected, but when a user notices the blocking dropDatabase and wants to inspect and kill the current operation, the user cannot use auto-completion. Otherwise, the shell hangs after the <tab>
Using maxTimeMS with listCommand could fix the usability issue.