Dynamic Mask...? :\

Here’s a fun little one that I hope someone can help me with.

May I draw your attention to http://www.mycomputermusic.net

As you can see, ladies and gentlemen, when you click on a triangle, a window opens up which display’s the site contents - however, you can still click triangles “behind” the window.

How would I stop this from happening, anyone have any ideas, please?

Thanks
jonny x