Try to open an empty scene and then edit your Prefab, because I think you are having problem when editing prefabs right?
Yep, that's right. I didn't notice that fact before.
Nevertheless, this morning all is fine again. I didn't do anything and I can edit that prefab without losing focus. Then again that's the first thing I've done after opening Unity, so I'm not sure if it's going to happen eventually.
I'll be sure to notify you guys and try to retrieve more data next time, so you could have better feedback to debug it.
also, make sure you have 1.8.3, the latest version from the Asset store. Do you get the problem even then?
I'm in 1.8.3 indeed. In fact I suspected about that recent version being the culprit because I recently upgraded to it.
Thank you all!