The tests are plagued by heisenbugs

Many tests will not work reliably. Some tests will always fail if you reload the page. Some tests will fail every second reload, and pass every second reload. All tests will however run if run in isolation. Trying to debug the issue (in any case including alerts or having a debugger) will make the tests pass.

I suspect timing issues, but adding delays seem to make no difference.

I suspect a JS/jQuery/qUnit guru is needed to look at this.

The issue names no files, test commands, or specific failing tests. Start by running the full test suite and reproducing failures across repeated page reloads, then compare those results with isolated test runs; done means the tests pass reliably both together and in isolation.

Written by the indexing model from the issue text.