This just in at 5:50 AM.
I'm up early for non-feline reasons, and Billy joined me in my office. He hopped up to the top shelf (usually Whitey's domain). Then Whitey came in the room, which made the door next to the bookshelf swing open and (almost) closed.
I turned around and Billy had both forepaws holding the top of the door, as if he was planning to either open the now interesting door (it moved) or take a walk on it.
With Whitey having rolled off this same shelf once already and Billy also tempting fate this week, the images I had of Billy falling and catching a door knob in the ribs or landing on his brother were not pretty.
The door has been swung fully open (and out of reach of inquiring paws) by your truly. I suspect Katherine would rather risk sleeping through the sounds of my typing rather than the sounds of a cat crashing seven feet to a hardwood floor.
Meanwhile, having written this up, I hear energic cat fu from the first floor. Sounds good to me.