Tufte running headers not using full width of page

I use Minion Pro and XeLaTeX together with the tufte class. There is an issue with caps in headers (as can be noticed if removing the two line marked %% headings fixes below). My fix is to use the myheadings option – but I get a strange effect where the running headers only use up the main text block width and not the full width (the main block + sidenotes).

\documentclass[nols]{tufte-handout}
\usepackage{fontspec}
\setmainfont{Minion Pro}

\title{Example of tufte headings problem}
\author{Author}

\markboth{\textsc{example of tufte headings problem}}{\textsc{example of tufte headings problem}}

\begin{document}

\maketitle

\section{First section}

First section body.

\newpage

Second page.

\newpage

Notice how the headings only uses the main text block width and not the full width of the page.

\end{document}

What could be a solution?

-

Tufte uses fancyhdr for headings, and your work-around is clobbering it. If you want to redefine the header manually, use the fancyhdr way of doing it (using \fancyhead).

But there is a better way anyway.

The root cause of your difficulty is that when loaded with XeLaTeX, the tufte class does not know how to construct its \smallcaps and \allcaps macros, which produce spaced small and large capitals. It's \smallcaps that is used in the heading, which is what produces your problem.

I think the better way to achieve what you want is to get those working, using fontspec. The following seems to work