Custom Finder sidebar icon gets stretched

Yesterday, I installed and configured Netatalk 3 on my Raspberry Pi in order to be able to mount it as an AFP share. Today I got fed up with how it looked in Finder's sidebar, so I set out to create an icon for it. I then made this (note the pixel-perfectness of the 16x16 version):

and combined the four resulting files (16x16, 16x16@2x, 32x32, 32x32@2x) using iconutil to get a retina-ready ICNS. Inside of Finder, the 16x16 version of the icon looks like it got shifted to the side by half a pixel:

Why is that? I double-checked the resolutions of my source icons and those inside the ICNS file and they're all correctly sized.

Solution 1:

Turns out that Finder uses 18x18px icons, not 16x16px icons. If you're designing sidebar icons, you'll need to supply these as well, which is impossible because there is literally no tool that'll pack 18x18 images (or their @2x counterparts) into an icns file.