Actually, you are not supposed to be able to edit a relationshipField, this is not supported by the feature. The empty fields you tried to edit are relationshipField and that’s why you could not edit it. I can confirm that empty String, Number, and Date fields are editable.
I’m not sure this is something we want to do actually. Double left click are not that usual on the web.
Anyway let me push your remarks in our product board, we might want to support relationShip fields in the future, so your feedback is really appreciated.