ICO should not wear a black halo
A clear PNG dropped into a composite icon often dirties the edge.
Light fills show a dark fringe. Dark taskbars show mud. 1.1.4 on 2026-01-06 premultiplies alpha and prefers native iconutil on macOS. 1.1.5 the same day tightened the Windows taskbar and Retina preview.
macOS uses native iconutil. Premultiplied alpha on clear edges. 16px taskbar must still read.
Two sizes to watch
16 px. Taskbar and title bar. Silhouette beats gradient. Use blacken / cutout if needed.
256 / 512 / 1024. Store and Finder. Detail may grow. Do not share hairlines with 16 px.
ICO / ICNS can hang off any platform config, not only Windows / macOS rows. See paths are the spec.
Failure falls back to the library. The file should still work. If you still see a halo, update past 1.1.4 and check the source is not already a dirty JPG.
Do not
- Do not use JPG as a clear icon source. JPG has no alpha.
- Do not judge ICO in Explorer thumbnails. Look at the taskbar.
- Do not treat a blurry Retina preview as a blurry ZIP. Update, then open the large PNG.
Next: see the device, guidelines.