I've no idea if they introduced an an ini setting for aliasing or a real solution for this but a classic, flat, square, button can be collapsed to 5px and, if you make the border transparent (which is 1px), it looks like a 3px line. However it won't do arbitrary angles-horizontal and vertical only.
You may have to rethink how you display your fluid with either the picture control or jpg/png images copy and pasted to the FP, if it's a real issue. I'm not sure how you are using to display a fluid (reveal behind a progress bar?). I've never used decorations as they are so limited.