(This bug happened on the latest build deployed Dec 10 on the internals demo system (version 12), as well as on previous.
It might be already reported but I could not find it)

Some time, and I can't figure out in which context, when you create a new content item with a richtext field, the online editor does not kick in. The filed is just like a regular textline.
The best is to check out this small video recording.