as of 1.2.0, causes "Disconnected, because no message in 10000 ms."

[rally25rs]

See: karma-runner/karma#2652

I tried this with both ChromeHeadless and with PhantomJS, both show the same behavior.

If you set up these package versions:

$ yarn list | grep karma
├─ grunt-karma@2.0.0
├─ karma-chrome-launcher@2.2.0
├─ karma-junit-reporter@1.2.0
├─ karma-mocha@1.3.0
├─ karma@1.7.0

You will get this error (I am on OSX Sierra)

21 08 2017 17:31:23.807:INFO [karma]: Karma v1.7.0 server started at http://0.0.0.0:9001/
21 08 2017 17:31:23.807:INFO [launcher]: Launching browser PhantomJS with unlimited concurrency
21 08 2017 17:31:23.810:INFO [launcher]: Starting browser PhantomJS
21 08 2017 17:31:24.949:INFO [PhantomJS 2.1.1 (Mac OS X 0.0.0)]: Connected on socket Lg-i7YrrAMRgssJKAAAA with id 53434843
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 197 of 319 SUCCESS (0 secs / 0.075 secs)
PhantomJS 2.1.1 (Mac OS X 0.0.0) ERROR
  Disconnected, because no message in 10000 ms.
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 197 of 319 DISCONNECTED (10.351 secs / 0.075 secs)

If you backdate to karma-mocha@1.2.0 it will still error, however 1.1.1 will work:

$ yarn add karma-mocha@1.1.1

21 08 2017 17:32:41.103:INFO [karma]: Karma v1.7.0 server started at http://0.0.0.0:9001/
21 08 2017 17:32:41.103:INFO [launcher]: Launching browser PhantomJS with unlimited concurrency
21 08 2017 17:32:41.107:INFO [launcher]: Starting browser PhantomJS
21 08 2017 17:32:42.275:INFO [PhantomJS 2.1.1 (Mac OS X 0.0.0)]: Connected on socket RHF27V5yNo9Z4VNUAAAA with id 38042808
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 246 of 319 SUCCESS (0 secs / 0.095 secs)
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 316 of 319 SUCCESS (0 secs / 0.152 secs)
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 317 of 319 SUCCESS (0 secs / 0.158 secs)
PhantomJS 2.1.1 (Mac OS X 0.0.0): Executed 319 of 319 SUCCESS (0.576 secs / 0.177 secs)

Done, without errors.