Can I detect the user viewable area on the browser?

Using the following, you can get the browser’s viewport size.

window.innerHeight;
window.innerWidth;

Refer to: http://www.javascripter.net/faq/browserw.htm

If you want to detect how far they have scrolled down the page, you can use

window.scrollX;   // Horizontal scrolling
window.scrollY;   // Vertical scrolling

Also, I have found a window object – window.screen. On my system it has the following data:

window.screen.availHeight = 994;
window.screen.availLeft = 0;
window.screen.availTop = 0;
window.screen.availWidth = 1280;
window.screen.colorDepth = 32;
window.screen.height = 1280;
window.screen.pixelDepth = 32;
window.screen.width = 1280;