(See similar issue from 2015: #1640)

Expected behaviour

✅ When restartOnFileChange: false, after a test run completes, saving a test file re-runs the test(s)
✅ When restartOnFileChange: true, after a test run completes, saving a test file re-runs the test(s)

Here I saved test.js three times, and it ran four times (initial run, plus 3 x saves):

Actual behaviour

✅ When restartOnFileChange: false, after a test run completes, saving a test file re-runs the test(s)
🚫 When restartOnFileChange: true, after a test run completes, saving a test file does not re-run the test(s)

Here I saved test.js four times, and it ran once (initial run only):

Steps to reproduce

I have created a trivial reproduction of the issue here:
https://github.com/scottohara/karma-restartonfilechange-bug