Hi,
the combobox.js doesn't provide a scrollbar.
You could try setting the 'native' setting to true to use the browser internal combobox as alternative (but there can be problems on mobile devices):
http://krpano.com/plugins/combobox/#native
The combobox plugin itself is unfortunately already a bit outdated, it works and shows differently in html5 and flash and it's difficult to customize. Therefore there is already a longer-term plan to replace it with a fully xml-based component (internally based on layers, scrollarea and textfields). With that it would work the same everywhere and it would be easier to extend and to customize. Making that should be basically already possible today, but internally I first want to bring textfield and scrollarea directly into the viewer instead of having them as external plugins. That opens more ways and makes nicer and better usage possible.
That means there are currently no plans to extend the combobox.js, but when the 'combobox.xml' will be there as replacement, then scrollbars should be also a part/feature of it.
Best regards,
Klaus