Thanks for confirming my sanity, albeit temporary.
Not addressing the sanity part (I have a meeting about you with some other white-coated fellows later on), but I am pretty sure this was raised before.
Even in the last week by a French forum member I believe. (I now can't find that post though)
I do recall it occurring a few times when I had both the tag editor docked at the bottom, and the vertical tag editor open.
But I can imagine that that is asking for some problems.
But it also happened with using only the docked tag editor.
Perhaps I will put some effort in trying to replicate it and focus on the the circumstances the coming week.