My navigation panel currently has the following attributes:
<div id='navpanel'
style='float:left; width:300px; height:100%; overflow:hidden;
background:URL(assets/img/bg_tr.png);'>
However as you can see in the screenshot below, the height does not fill out until the end of the screen. I have tried to switch to “auto” however with the same result.
Does anybody have an idea how I can get the height of my panel to go down until the end no matter what screen resolution I currently have? Some advice would be highly appreciated – thank you.

Try adding this:
http://jsfiddle.net/AnsJy/1/