Shifting underbrace with lualatex and unicode-math

Question

Compiling this example:

\documentclass{scrartcl}

\usepackage{mathtools}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}

\begin{document}
\begin{equation*}
  \symcal{L}_{\text{QED}} =
    - \underbrace{\frac{1}{4} F_{\mu\nu} F^{\mu\nu}}_\text{Photon}
    + \underbrace{\bar{\psi} (i \gamma^\mu\partial_\mu - m) \psi}_\text{Dirac}
    - \underbrace{eQ\bar{\psi} \gamma_\mu \psi A^\mu}_\text{WW}
\end{equation*}
\end{document}

using lualatex from TeXLive 2016, the second \underbrace is shifted to the left. This only seems to happen when the first thing in the \underbrace is \bar.

Even adding something ridiculous like \hphantom{} resolves the issue.

Only \underbrace{\bar{\psi} (i \gamma^\mu\partial_\mu - m) \psi}_\text{Dirac}:

Adding an \hphantom{} before \bar

---

A “more minimal” example, to be compiled with luatex

\input ifluatex.sty
\ifluatex
  \input luaotfload.sty
\fi

\font\mf="[latinmodern-math.otf]"
\textfont1=\mf \scriptfont1=\mf \scriptscriptfont1=\mf
\protected\def\underbrace#1{%
  \mathop{\Umathaccent bottom 7 1 "023DF\relax {#1}}\limits
}
\protected\def\hat{\Umathaccent fixed 7 1 "00302\relax}

$\underbrace{z+w}_y$\qquad$\underbrace{\hat{z}+w}_{y}$

\bye

If compiled with xetex, the output is

Changing the math font to xits-math.otf doesn't change the situation. With Asana-Math.otf the output is different:

Answer 1

As a temporary workaround, you could add the empty brace group proposed by David Carlisle in the definition of the over- and underbraces.

\documentclass{article}
\usepackage{amstext}
\usepackage{unicode-math}
\ExplSyntaxOn

\cs_set:Nn \__um_set_math_overunder:Nnnn
 {
  \cs_gset_protected_nopar:Npx #1 ##1
   {
    \mathop
     { \__um_accent:nnn {#2} {#3} {#4} {{}{##1}} } % see the empty braces?
    \limits
   }
 }

\ExplSyntaxOff
\begin{document}
\[
  \symcal{L}_{\text{QED}} =
    - \underbrace{\frac{1}{4} F_{\mu\nu} F^{\mu\nu}}_\text{Photon}
    + \underbrace{\bar{\psi} (i \gamma^\mu\partial_\mu - m) \psi}_\text{Dirac}
    - \underbrace{eQ\bar{\psi} \gamma_\mu \psi A^\mu}_\text{WW}
\]
\end{document}