Folks, I received a polite and helpful technical explanation of how smartsheet works from one of their managers. Without permission to say more, just for your technical interest it's probably okay to pass on that it's an AJAX app run by JavaScript generated by an internal framework they created before JQuery and ExtJS were available. So the answer is ... as we suspected ... it's implemented using the general GWT and ExtJS technique of being a gigantic illusion created by JavaScript.
Greg
