diff --git a/kstyle/breezehelper.cpp b/kstyle/breezehelper.cpp
--- a/kstyle/breezehelper.cpp
+++ b/kstyle/breezehelper.cpp
@@ -1258,7 +1258,7 @@
         painter->setRenderHint( QPainter::Antialiasing, true );
 
         const QRectF baseRect( rect );
-        const qreal radius( 0.5*Metrics::ScrollBar_SliderWidth );
+        const qreal radius( 0.5 * std::min({baseRect.width(), baseRect.height(), (qreal)Metrics::ScrollBar_SliderWidth}) );
 
         // content
         if( color.isValid() )