Some characters do not combine correctly with cá and nháy

[guocuozuoduo]

A test was done by picking out the characters with "cá" and "nháy" marks in the PUA and recreating them using the combining marks. A number of characters failed to combine correctly.

I have excluded the characters whose base characters (without cá or nháy) are not yet in Unicode.

I have highlighted in grey those characters that visually have such parts, but where I am uncertain if those parts are indeed those marks and not part of the characters themselves, or if the base characters are cognate. If they are not intended to be such, they can be ignored. Additionally, highlighted yellow and blue are characters that could use variation selectors, but currently are not implemented.

Finally, there are a large number of characters with what appears to be an alternate form of nháy (the mark itself is mapped to F1F09), and some share a base character with the "default" nháy, while others are unique.

Would it be possible to use the combining nháy to map to these characters, or should this be treated as a separate diacritic not yet in Unicode?

[nomfoundation]

We have only added layout rules for cá and nháy where we have found documented examples. For example, U+F06DB (⿰於𡿨) is in the font, but we don't have a rule since we haven't found an actual example used in text. It looks like most of the missing rules in this issue are due to this limitation. If there is a need, we can add them. The alternate nháy forms are not yet supported. Unicode does not provide a way to vary the shape of marks, so we either need to encode a variant nháy form, or encode the characters composed with the variant nháy. There are probably about 150 such examples.

[nomfoundation]

v5.16 now supports all of the unambiguous cá nháy. We still have no solution for the alternate forms.