[luatex] Placement of subscripts with the STIX fonts v 2

[Harald Hanche-Olsen]

Consider this code:

\documentclass{article}
\RequirePackage{fontspec,unicode-math}
\setmainfont{STIX Two Text}
\setmathfont{STIX Two Math}
\begin{document}
$\gamma_{e}\ne\gamma_{\mathrm{e}}$
\showthe\textfont4
\end{document}

When I run this in the frozen TL2017 (LuaTeX version 1.0.4),
the subscript on the left gets a negative kern, tucking it in under the gamma.
This does not happen to the one on the right.

That negative kern comes from the font (character 12057).
The font STIX2Math.otf is Version 2.00 b137 according to otfinfo.

In a recent exchange with Ulrike Fischer on the tex.SE chat,
I have learned that this also happens with LuaTeX 1.0.6 on TL2017.
On TL2018, however, she gets no kern at all on either side.

This seems wrong. Perhaps there was a change between LuaTeX 1.0.7 and 1.0.8,
or it could be related to changes in unicode-math.sty.

Replacing \mathrm by \symrm helps (on TL2017), so I don't have an immediate
problem with this.

FWIW, the example works as expected with XeTeX.
(Still on TL2017. I don't have TL2018 installed.)

– Harald
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