Feature Request: Force undefined characters to map to `.notdef` in cmap

I am creating kana-only fonts with Glyphs 3.

At the moment, when a user types Kanji in Illustrator (or other apps), the missing characters are not shown as .notdef (tofu). Instead, the application substitutes glyphs from another font.

What I would like to achieve is:

  • All undefined characters (for example, CJK Unified Ideographs) should be explicitly mapped in the cmap to cid00001 (the .notdef glyph).
  • This way, any Kanji or unsupported characters will always display as tofu from the font itself, regardless of the application’s fallback behavior.

It would be very helpful if Glyphs could provide an option to generate such a cmap automatically, mapping specific Unicode ranges (like U+4E00–U+9FFF) or all undefined code points to .notdef.

Would it be possible to add this feature in a future update?
I believe it would be very useful for designers who want to distribute kana-only fonts and ensure consistent tofu display across different applications.

By the way, the kana fonts from major Japanese font vendors do this processing. If there are no undefined characters, tofu will be displayed.

The top font contains only kana glyphs exported from Glyphs.

The bottom font contains only kana glyphs released by a major Japanese font vendor.

When typing with this font in software like Illustrator, it automatically converts to kanji like the top example and replaces it with another font containing kanji, making it extremely difficult to use.