I'm:

Clicking to move thread
Focusing the target select box without choosing anything(e.g. tabbing or just doubleclick at select)
Using only keyboard to change value to any different than current (e.g. I just typing "Ric" to get "RichFaces" there)

"Move Thread" button still disabled. Even if you will change value using keydown or keyup it will remains disabled. And it will become enabled only after I will select it via mouse.

Checked under Chrome only. Not sure if this specific to Chrome or common.