Move a combining character with both \raisebox and \kern

Question

The font I'm using, Linux Libertine O, is not able to properly combine a circumflex (U+0302) with a preceding vowel symbol that has a macron (e.g. U+014D):

\documentclass[17pt]{extarticle}
\usepackage{fontspec,xunicode,xltxtra}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont{Linux Libertine O}
\newcommand{\schleifton}{\char"014D\char"0302}

\begin{document}
\textit{*-{\schleifton}z}.
\end{document}

I try to raise the circumflex with \raisebox:

\documentclass[17pt]{extarticle}
\usepackage{fontspec,xunicode,xltxtra}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont{Linux Libertine O}
\newcommand{\schleifton}{\char"014D\raisebox{.3ex}{\char"0302}}

\begin{document}
\textit{*-{\schleifton}z}.
\end{document}

The circumflex is too far to the right, so I try to move it closer to the preceding character with \kern. The problem, as you can see, is that it also moves the following character (the 'z') to the left. How should I fix this?

\documentclass[17pt]{extarticle}
\usepackage{fontspec,xunicode,xltxtra}
\defaultfontfeatures{Mapping=tex-text}
\setmainfont{Linux Libertine O}
\newcommand{\schleifton}{\char"014D\kern-.1em\raisebox{.3ex}{\char"0302}}

\begin{document}
\textit{*-{\schleifton}z}.
\end{document}

---

As egreg points out in his answer, it is necessary to use \kern a second time to undo the leftward shifting of characters. He also points out, however, that using these commands will have slightly different outcomes for regular and slanted fonts. He suggests one way of taking care of that. Below, I have decided to just specify the regular and the slanted character separately:

\documentclass[17pt]{extarticle}
\usepackage{fontspec, xunicode, xltxtra}
\defaultfontfeatures{Mapping = tex-text}
\setmainfont{Linux Libertine O}
\newcommand{\schleifton}{\char"014D\kern-.11em\raisebox{.15ex}{\char"0302}\kern.11em}
\newcommand{\schleiftonitalic}{\char"014D\kern-.08em\raisebox{.25ex}{\char"0302}\kern.08em}

\begin{document}
\Huge{*-{\schleifton}z}.
\Huge\textit{*-{\schleiftonitalic}z}.
\end{document}

Answer 1

You need to kern also after the inserted accent:

\newcommand{\schleifton}{%
  \char"014D\kern-.1em\raisebox{.3ex}{\char"0302}\kern.1em }

However, by using a more complex definition, you can get a kern that takes into account also the slant of the current font. Here it is, with some samples.

\documentclass[17pt]{extarticle}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{Linux Libertine O}
\makeatletter
\def\schkern#1{\kern#1\dimexpr0.11em-\strip@pt\fontdimen1\font\dimexpr0.1em\relax\relax}
\makeatother
\newcommand{\schleifton}{\char"014D\schkern{-}\raisebox{.3ex}{\char"0302}\schkern{+}}

\begin{document}
\textit{*-oz}.

\textit{*-{\schleifton}z}.

\schleifton

\Large

\textit{*-{\schleifton}z}.

\schleifton

\end{document}

What do we do? The idea is the same you had, but refined.

In \fontdimen1\font (Xe)TeX keeps the slant factor of the current font as a dimension in unit points, so with \strip@pt\fontdimen1\font we get the slant factor, let's simply call it σ.

We need to back up a bit, but then to advance a smaller bit, in order to take the slant into account. Thus the amount of the main backing up (found by trial and error) is 0.11em; the smaller amount turns out to be good as 0.1em multiplied by σ. Thus we back up by

0.11em - σ * 0.1em

In order to perform the second multiplication, we need to transform 0.1em into a TeX dimension, which is obtained by \dimexpr0.1em\relax. Thus we can write

0.11em - σ\dimexpr0.1em\relax

but in order to feed \kern with it we need another \dimexpr:

\kern\dimexpr 0.11em - σ\dimexpr0.1em\relax\relax

But we need also to undo the kerning after the accent, so \schkern is defined with an argument which is either + or -, the latter for backing up:

\def\schkern#1{\kern#1\dimexpr0.11em-\strip@pt\fontdimen1\font\dimexpr0.1em\relax\relax}

Thus \schkern{-} backs up, while \schkern{+} pushes forward.