<p>How can I add horizontal scroll capabilities to the asp.net listbox control?</p>

If you really, really need it, one idea would be to create a custom
ListBox class whose HTML looks like this: sets the width of the SELECT
to that of your widest value's width (the max width of the scrollbar,
for example). Now wrap that SELECT inside of a DIV of the 'constrained'
size and let it scroll on overflow.
Here's a quick example starting down those lines, here's the type of
HTML you want spit out by a control:
```
```
<div style="width:200px; height:100px; overflow:auto;">
<SELECT size="4">
<OPTION
Value="1">blahblahblahblahblahblahblahblahblahblah blahblah</OPTION>
<OPTION Value="2">2</OPTION>
<OPTION Value="3">3</OPTION>
<OPTION Value="4">4</OPTION>
</SELECT>
</div>
```
```
so in essence I'd recommend creating a composite custom control for
this, which renders this HTML. They're pretty easy to make, Google on
the terms 'composite control asp.net'.
The toughest part will be matching up the div dimensions to that of the
select box, to make the scrollbars work/line up properly. That's why
it's kinda tricky.
Source
Also, take a look at this:
Automatically Adding/Hide Horizontal Scroll bar in the ListBox control
EDIT
: Make sure you have enough height to include the scroll bar height or else you'll get the vertical scroll bar on both controls.