WebApp: Incorrect Script Running Halo

When a block is clicked to run it, it gets the green halo. And when clicked again the halo goes away and the execution stops. In the browser version, a quick succession of clicks causes this relationship to be broken:

Things execute with a green halo, and then if clicked again quickly, the halo goes away, but the program keeps running.

This must be a synch issue between the browser and the MB code.