Glyphs for characters in other blocks incorrectly falling back to system font

This issue is applicable to Adlam and N'Ko.

When text contains a punctuation mark such as an Arabic question mark, the glyph displayed should be that contained in the Adlam or N'Ko font, if it contains one.

The Gap

At least one browser on desktop and mobile render the Arabic question mark (and perhaps other punctuation) using a fallback font despite the font containing the appropriate glyph.

Example in N'Ko:

This was tested with the Noto Sans NKo font. In Gecko and Blink the question mark falls back to a different font.

It may be related to a script itemization issue where Adlam or N'Ko is not properly recognized. A request was made to the Unicode Consortium to fix the Script Extensions table.

Other characters from the Arabic and Devanagari script blocks exhibit the same behaviour. For example, a danda used in Limbu, or a tatweel used in Syriac.

More:

• GitHub issue

Priority

A question mark is visible, just not in the correct font. So marking as advanced.

Tests

• Interactive test, If an N'Ko or Adlam font contains Arabic punctuation, the glyphs rendered should be from that font, and not fall back to another font
  

Action taken

Gecko

The Unicode properties & algorithms group proposed an update to Script_Extensions for Adlam and N'ko to include the appropriate Arabic punctuation marks, and it was discussed in UTC. See PRI428g in https://www.unicode.org/L2/L2021/21069-utc167-properties-recs.pdf

Outcomes

The Unicode Consortium has fixed the script extensions table.

All 3 browser engines now do the right thing.

Start with the linked interactive test in w3c/glyph_character_tests issue 7 and review the Gecko Bugzilla issue 1834316, then check the Unicode Script_Extensions table changes cited in the issue. Done is confirmed when Adlam and N’Ko punctuation uses the appropriate font across all three browser engines; the issue reports that outcome already.