When warp is on and one moves the focus to an empty frame, the mouse doesn't move. This seems strange to me. I would expect either memory for the frame (but that's gnarly, since the window is the thing that has remembered mouse state) or the mouse to move to, say, window center or something.
This comes up because if focus policy is mouse-based, then warp must be on so that keyboard focus changes aren't undone by the mouse still being in the old frame. But then it becomes important, if the focus is changed to an empty frame, that the mouse follows.
This can be sorted in a number of ways, but the most logical to me is to make focus change to an empty frame actually warp the mouse to the empty frame (if warp is on).