Adding tests to a test-execution can't be done with the mouse

If I want to add a test to a test-execution, I klick on "+ Add" --> a dropdown with "Tests..." and "Testsets..." appears.

I am not able to klick on Tests... or Testsets... (if I click on one of them, the dropdown simply disappears without the selection being highlighted and nothing happens)
Instead I have to use the keyboards arrow-keys to select Tests... or Testsets... and hit Enter/Return key.