The resize handles always stick to the edges of the widget's cavity, even if the widget is too small to fill the entire space. Before we can make the frame grow to fill its cavity, we must make a column resizable inside the frame. Click on the frame so that the row and column handles are red, and click on the column handle to make sure it's resizable. Then click once on the frame and it will have the resize handles again.
Also notice the column grid lines of the background grid are not drawn through the cavity.