Horizontal conjunct with u-matra places u matra on first member, but should be on second

[xadxura]

Here is the problem string: द्यु dyu

In Nirmala UI font this renders as a horizontal form with explicit virama because Nirmala UI is optimized for Hindi and low descenders.
In Chrome and Firefox, but not in Windows and Edge, the u-matra (ु) on the ya (य), appears to be positioned below the da (द). Therefore readers see duya rather than dyu.

[behdad]

Ok I see what's going on. Our base-selection is fine.

In this font, the virama,ya first forms a ligature, then decomposes back to virama,ya. This causes those two to be marked parts of a MultipleSubst sequence. When attaching the matra, we look for the first of the MultipleSubst sequence because that's where we attach to (because of eg #740). In this case, the first glyph in the MultipleSubst sequence is a mark, so we skip it and attach to the base char before it.

I'll add some more checks to fix this.