>
> Check out the character composition rules in thai-util.el, perhaps
> something is missing there.
>

Thanks for the pointer.  With

58c58,59
< (let ((l '((?ก consonant "LETTER KO KAI")				; 0xA1
---
> (let ((l '((?◌ consonant "DOTTED CIRCLE")
> 	   (?ก consonant "LETTER KO KAI")				; 0xA1

and a reevaluation of the relevant s-expression, thai-compose-region
and thai-compose-buffer give me almost correct results for that test
case.  The vowel does combine, but renders oddly:

When the cursor is over the dotted circle, the vowel, which should
appear roughly centered above, is shown above and to the left.

When the cursor is just to the right of the dotted circle, the vowel
is shown in the same place, but the character to the left of the
dotted circle is occluded by another dotted circle.

When the cursor is just to the left of the dotted circle, the vowel is
shown above and to the right.

When the cursor is elsewhere, the rendering is unchanged from its most
recent state.

Perhaps that's a separate issue?