Understanding issue with CSS width property

In side a wrap div of width 800px, I have a sidebar div with the following code:

float:left;
width:50%;
background-color:white;
font-size:1em;
position:fixed;

The sidebar won't take up 50% of the wrap (400px) but 50% of the browser window (1350px). *Why?*

The div called main sits next to the sidebar on the right with width 50% and it is around 400 px.

 

Probably because of {position: fixed;}. Its containing block is the viewport.

cheers,

gary

 

That's it!

Thank you very much.