Dear experienced Skinners,

I'm trying to build my first skin and I can't seem to figure out this problem:

As you can see, the transparent part of my window somehow gets this blurred effect where it's supposed to be fully transparent. Also Note how the blur has rounded corners, while my image is of course a rectangle and any of my shapes that are not supposed to be transparent near that corner also don't have any rounded corners.

I tried to google the issue, but didn't get any hits, though I assume this is just a simple beginner's mistake? Does anybody know what I'm missing and doing wrong'