> Note that the latter control is rebuild (the cell is re-run) on every change to focus But it isn't? Because the cursor cell is never invalidated, Observable has no reason to recompute play or any of the other controls.

Hmmm, maybe my explanation is not clear enough (well, re-reading, it is not clear at all :P), or maybe I don't understand your point. What I mean is that, as MyScrubber only implements a setter on the value, but not on the min and max options, I have to delete and create again the embedded scrubber on every change of the min (focus[0]) and max (focus[1]) controls

Ah, understood. I'd remove "(the cell is re-run)" from the text, then it should be fine (perhaps also refer to the "Play" control instead of the "latter control"?). Also, play should probably be "viewof play"?

Thanks for the comments. I updated the text