It was getting in the way of native styled background. In particular, it was turning rounded Breeze corners into visually broken "korners".
| Before | After |
|---|---|
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There's still some overlapping from delegates: neither their background nor content is masked with a scrollview frame, so they are only being clipped by the rectangle bounds of a scrollview as an Item. But imho this is already an improvement, and I didn't spot any regressions in any of default themes (Breeze, Oxygen, Fusion, MS9x) and light/dark color schemes. Checked all System Settings pages with cherry-picked KF5 patch.
CC @teams/vdg