Funny, after reading this thread, I had this happen to me last night. When I tried to move the library window around after making it floating, the right side 'stuck' to the right of the screen and only resized larger when I tried to move it. When I let it go, it docked above the doc window as in the OP's screenshot above. Messing around with it, I wound up with a tiny doc window and had to go into preferences to set poser back to default. That fixed it for me, but I don't think I'll be doing much floating with the ui anymore, even though it's cluttered, imo.