[6.x] Combobox: Prevent options being cut off & add scrollbar

[duncanmcclean]

This pull request fixes an issue where options might get cut off in a Combobox dropdown. It also adds a scrollbar to improve usability when there's lots of options:

CleanShot.2025-11-28.at.11.50.41.mp4

Unfortunately, because we need to virtualize the options to help with performance, it means that only the visible options actually exist in the DOM.

This means we can't rely on the browser's built-in scrollbar, we have to build our own. It's a little bit of a pain, but 🤷‍♂️

Closes #13205

[jackmcdade]

Was the Reka Scrollbar not an option here? Otherwise, looks and works great on my end!

[duncanmcclean]

Was the Reka Scrollbar not an option here? Otherwise, looks and works great on my end!

I looked into Reka's ScrollArea component, but unfortunately, it doesn't work alongside virtualisation because it relies on the browser's native scroll stuff under the hood.

With virtualization, only a handful of options actually exist in the DOM at once.