Steps to reproduce:
Start new question/answer or edit one of your existing.
Type something and make sure you can highlight it with the mouse.
Hover over your user name and wait for the box to expand.
Get back to the text you typed and try to highlight it now - no matter how. (keyboard or mouse)
The selection disappears immediately and you effectively can't highlight anything anymore.
While annoying, it also cause I can't use the editor icons or shortcuts at all, e.g. attaching links and even make text bold if you don't know about the
**bold here** stuff.