I have a div with position:fixed, as the image below suggests.
I want it to be fixed vertically, but I want it to scroll horizontally with the rest of the content (notice the horizontal scroll bar).
Is this possible with CSS? Or do I need Javascript (in this case, how could I do it)?

Old question, but I was also looking for a solution also, and found a nice simple jquery solution, and figured someone else might find it helpful if nothing else:
with header being the div, and originalLeft being, well, the initial left:position. This also works if someone scrolls and then zooms out, such that it would go where you would want it to.
You need javascript. Here you can read a tutorial (with mootools): http://www.rickyh.co.uk/css-position-x-and-position-y/. Here a similar question, solved with jquery: CSS: fixed position on x-axis but not y?
you can do it by css itself:
<div style="width:250px; height:250px; border:solid;overflow:scroll; overflow-y:hidden"> <div style="width:500px; height:500px; border:solid;"></div> </div>