Bug 402578 changed the click-in-the-scroll-trough behavior to match the current QWidgets default, but ideally we should respect the style hint so that when Bug 379498 gets fixes, all the scrollbars instantly react accordingly.
Sounds like a simple change to qqc2-desktop-style:
That attribute is already exposed via KQuickStyleItem::styleHint, so effectively one would just need to check style.styleHint("scrollToClickPosition") in the MouseArea of ScrollBar.qml and then jump to position like when doing a middle mouse click.
*** Bug 425825 has been marked as a duplicate of this bug. ***
A possibly relevant merge request was started @ https://invent.kde.org/frameworks/qqc2-desktop-style/-/merge_requests/31
Git commit 7b7946e110afae7b2364fa46c2cb874f2e501204 by Bharadwaj Raju.
Committed on 26/09/2020 at 14:57.
Pushed by ngraham into branch 'master'.
Respect scrollbar click to jump to position setting
Checks style.styleHint("scrolltoClickPosition") and respects the
behaviour set (whether clicking on a scroll bar position should move a
page up/down or jump to that absolute position)
M +14 -5 org.kde.desktop/ScrollBar.qml