Accents and unicode-math

Question

Why are certain accents in math mode invisible? I am using unicode-math and compile with XeLaTeX. Here is a MWE:

\documentclass{scrartcl}
\usepackage{unicode-math}
\begin{document}
x̃ŷz̄ $x̃ŷz̄$
\end{document}

This produces accents on the text mode letters but not on the math mode ones. Strange!

Answer 1

There is no Unicode “LATIN SMALL LETTER X WITH TILDE” so you get “x̃” by typing x followed by U+0303 “COMBINING TILDE”.

This can't work in math mode, which strictly works from left to right and a following character doesn't influence what's before it.

You can make existent Unicode points such as “LATIN SMALL LETTER O WITH TILDE” to behave like \tilde{o} in math mode with a trick such as

\AtBeginDocument{\mathcode`õ="8000 }
\begingroup\lccode`~=`õ
\lowercase{\endgroup\def~}{\tilde{o}}

One can think to do something like this for x (doing a lookahead to see if a combining character follows) but it would be slow and not particularly robust. It's really better to make your intentions clearer by typing

\tilde{x}

Just as a proof of concept, here's how you could manage x̃ (I also left the precomposed õ). The \@ifnextchar test should be extended to the other needed composite characters.

\documentclass{scrartcl}
\usepackage{unicode-math}

\AtBeginDocument{\mathcode`õ="8000 }
\begingroup\lccode`~=`õ
\lowercase{\endgroup\def~}{\tilde{o}}

\AtBeginDocument{\edef\mathcodex{\Umathcharnum\the\Umathcodenum`x }}
\AtBeginDocument{\mathcode`x="8000 }
\begingroup\lccode`~=`x
\lowercase{\endgroup\def~}{\addaccentx}

\makeatletter
\newcommand\addaccentx{%
  \@ifnextchar ^^^^0303{\tilde{\mathcodex}\@gobble}{\mathcodex}%
}
\makeatother

\begin{document}
x̃ŷz̄ $õx̃$
\end{document}

Answer 2

Based on egreg's answer:

\documentclass{scrartcl}

\usepackage{unicode-math}
\usepackage{amsmath}

\AtBeginDocument{\mathcode`x="8000 }
\begingroup\lccode`~=`x
\lowercase{\endgroup\def~}{\addaccent{x}}

\makeatletter
\newcommand\addaccent[1]{%
  \@ifnextchar ^^^^0303{\text{\emph{#1̃}}\@gobble}{%
  \@ifnextchar ^^^^0302{\text{\emph{#1̂}}\@gobble}{%
  \@ifnextchar ^^^^0304{\text{\emph{#1̄}}\@gobble}{%
  \text{\emph{#1}}}}}%
}
\makeatother

\begin{document}
x̃ŷz̄ $x̃x̂x̄ \frac{1}{x̃} $
\end{document}

I redefined the function to take the letter as an argument (that will make it easier to extend the code to all letters) and added more accents.

To enter text mode is probably not the most correct way to do it, but I found no other solution.