Upstream: https://github.com/karma-runner/karma/issues/1321
It seems there is an issue with using local web browsers concurrently. They share an Xvfb, so that could be a cause.
I'm not sure it is related to focus since I can run the tests locally at the same time without an issue. And in cases where this fails, often both builds fail. Not just the one that started first or last in tha overlap.