Cairo
Cairo copied to clipboard
Spacing symbols are treated as combining marks
The current font treats the spacing symbols U+FBB2 through U+FBB9, U+FBBD, and U+FBBE as combining marks. For example, here is <U+062F, U+FBB2> (د﮲):
It should look more like this:

@dscorbett I'm not sure I understand the problem
<U+062F, U+FBB2> generates the letter Thal ذ
and the dot is anchored on top of the letter as it should be

U+FBB2 is not a combining mark. It is a spacing symbol meant for describing glyph components, not for combining with letters to make new letters.
That is a Glyphs bug (specially fonts built with old versions of it), it used to use the dot symbols as combining marks for building composites. A simple fix would be removing the Unicode code points assigned to these glyphs in the source file and re-exporting the font.