Scrollbar deletes part of styled parent widget border

The attached picture shows the problem. The horizontal scrollbar deletes the right border of the styled parent widget and the vertical scrollbar deletes the bottom border of the styled parent widget. It does not matter if the scrollbar uses stylesheet. Also if scrollbar is not styled via stylesheet, the border of the styled parent widget is deleted. Here is the stylesheet for the dock widget in the picture:

QDockWidget > QWidget
{
border: 1px solid #7f7f7f;
border-top: none;
background-color: #333333;
}