BezierDammit

gistfile1.m

    NSColor *topBorderColor  = WHITE;
    NSColor *btmBorderColor  = HEX_COLOR(@"aaa");
    
    NSBezierPath *topBorder = [NSBezierPath bezierPath];
    [topBorder moveToPoint:CGPointMake(0.0f, 0.0f)];
    [topBorder lineToPoint:CGPointMake(dirtyRect.size.width, 0.0f)];
    [topBorder setLineWidth:1.0f];
    [topBorderColor setStroke];
    [topBorder stroke];

    NSBezierPath *btmBorder = [NSBezierPath bezierPath];
    [btmBorder moveToPoint:CGPointMake(0.0f, dirtyRect.size.height)];
    [btmBorder lineToPoint:CGPointMake(dirtyRect.size.width, dirtyRect.size.height)];
    [btmBorder setLineWidth:1.0f];
    [btmBorderColor setStroke];
    [btmBorder stroke];