The bug
Scrolling the superflat presets list only works when the list (respectively a list item) is focused. For all (?) other scroll lists you just have to hover over them, additionally for this specific list it used to work the same in 1.12.2.
Linked issues
is duplicated by 3
relates to 3
Comments 7
Can't say it won't scroll when unfocused on every instance, but I still experience times where it does not scroll (even when it is focused!).
Please update this ticket; it still affects 1.13-pre4!