D10080: ensure the rounded rectangle of scrollbard stays round

[Marco Martin]

mart added a reviewer: hpereiradacosta.

REPOSITORY
  R31 Breeze

REVISION DETAIL
  https://phabricator.kde.org/D10080

To: mart, #plasma, #vdg, hpereiradacosta
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart