Comment 8 for bug 1016763

It's not quite the hairline feature you're asking for, but I created a patch for a new display mode that makes "hairlines" (anything smaller than half a pixel on screen) more visible: https://gitlab.com/inkscape/inkscape/merge_requests/365

I also submitted a patch to win32 Cairo so that it properly sends anything smaller than the device's smallest unit size as a hairline (0.001" works) so you can print directly to laser printers now: https://lists.cairographics.org/archives/cairo/2018-October/028767.html