If you're only seeing a corner of the test area, it's likely that your browser is zoomed out. This displays the page smaller so you can see more on one screen, but causes problems with Flash since Flash content doesn't zoom. As a result, our test stays the same size but has a smaller area to display in, meaning you'll only see a corner. If you're accidentally zoomed in, there may appear to be extra empty space to the right and below the test window.
To fix this problem: you'll want to set your zoom back to "normal" size.
Instructions on how to adjust the zoom on most popular browsers: