I'm talking about behaviour that can be switched off with --no-exit option.

Having this as default looks dangerous, as it may hide serious issues in tests (any orphaned async flow is just silenced, leaving no clue to developer)

e.g. on my side I had a promise with crashing test not returned properly. Still test process as handled by mocha was exiting smiling with a success. It took me a while to figure out why it happens.