TIP: To see the whole grid and the pop-up window at the same time, press [F11] on your keyboard to switch to full-screen mode, then scroll the page down so that the grids just fits in at the top of the page. Then hit 'Run All Start Grid', then 'Solve by Steps'. To get out of full screen mode hit [F11] again.
NOTE: A number of people have commented that when you hit 'cancel' it doesn't work straight away. This is because the solver is deep down in the depths of solving (for the technically minded, recursion) and if it stopped straightaway this would mean that the solution grid would be corrupted. The solution: Just keep hitting cancel until it surfaces! As an alternative you can hit ALT+F4 to quit immediately, although this may mean you need to reset the solution grid before continuing.