Drawing a 1px line on both 1x and 2x screens.

1pxline.m

// A 1pt rect along the bottom of the view
NSRect bottomLineRect = NSMakeRect(0 , NSHeight(self.bounds) - 1, NSWidth(self.bounds), 1);


// Convert to the backing rect, and make sure that it is only 1px tall.
NSRect backingRect = [self convertRectToBacking:bottomLineRect];
backingRect.size.height = 1;
bottomLineRect = [self convertRectFromBacking:backingRect];