When drawing a 1.0 width cosmetic line on a Retina display, one would expect that line to occupy 2 real screen pixels, based on the 2x scale factor purportedly in use in Qt 4.8.4. The attached file and enlarged screen capture show that a 1.0 width cosmetic line is rendered with an actual 3 screen pixel width (the black line overlay is to make it easier to count).

QPainter.fillRect seems to not be effected by this, and so code which draws a line to the right of a filled rect will overlap by 1 screen pixel on a retina display.

On a more subjective note, the width of 3 screen pixels looks far too large for single-width line. Adjusting the pen width to ~0.55 will yield the expected 2 screen pixel width in some, but not all cases. This looks much better, but is non portable.