don't use a rounded rectangle when drawing the background of a widget

src/Widget.cpp

@@ -139,7 +139,7 @@ void Widget::handle_repaint_rect(RepaintRectEvent &event) {
         } else {
             painter.source_rgb(m_style->background_norm());
         }
-        painter.rounded_rectangle(event.box(), m_style->border_radius());
+        painter.rounded_rectangle(event.box(), 0.0);
         cr->fill();
     }