The bug
Scrollbar, sliders and buttons in the Customized World Settings are non-functional after you reset the settings back to default, unless you're looking at the list of presets. You can make them responsive again if you press "Randomize"
Related issues
Comments

Cannot reproduce either. Scrollbar works fine for me, are you using mac OS?
Edit: Confirmed.

I think he mixed up something here. I can use the scrollbar everywhere in the customized world settings except in the presets list if not focused.

MIchael, at first I didn't realise you had to reset the settings back to default to trigger the bug.

I'm aware you updated your ticket. When I wrote the last comment, I wasn't able to reproduce it right after I reset the settings back. What OS are you using?

Windows 10

Weird. Anyway. I resolved MC-123802 as 'invalid' as everything is now covered by this ticket here now that you have updated it.
Cannot reproduce?