如何将渲染剪裁为圆角边框？

I'm trying to create a border with rounded corners and a highlight on its top half. I'm using an ellipse with a radial gradient, overlapping the top half of the border, to give the highlight, but I can't figure out how to prevent the ellipse coloring the corners of the border. Here's a screenshot from Kaxaml:I'm trying to create a border with rounded corn