"block" in the diagramming sense, not as in an HTML div. Each frame consists of handling the mocked-up mouse events, adjusting the model of the diagram, including adjusting any lines that are attached to that block, then re-rendering as much of the page as necessary to reflect those updates. There's actually a LOT of javascript being run there, a fair amount of canvas rendering, text reflowing, DOM manipulation, and so forth. In other words, all the stuff that makes a real web app run these days.