Comment 5 for bug 448286

Actually, a "non-scaling" line thickness is not supposed to remain constant when changing zoom level. It has a fixed thickness only with regard to scaling operations while zoom level is involved in the visualization of the document.

So, a "hairline" will have a, let's say, 1 pixel width for visualization and is indeed a "non-scaling" for visualization (i.e. zoom level) but should be a 0 width for calculations (hence there's no point in specifying if it's "non-scaling" or not for calculations).

It's important not to make confusion between the document's unit and the visualization device's unit: 1 pixel is always 1 pixel on screen but it's not in an Inkscape's document: if you change the zoom level you can have 1 (Inkscape's) pixel that's 4x4 or 0,27x0,27 (LCD's) pixels on screen.

The attributes "hairline" and "non-scaling" refer to different and disjointed scaling scenarios (and needs).