tuftex-fullwidth.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{tuftex-fullwidth}[2023-06-20 package tuftex-fullwidth]

\RequirePackage[strict]{changepage}
% uses adjustwidth from changepage package

% Compute lengths used for full-width displays
\newlength{\@tuftex@overhang}
% used by the fullwidth environment and the running heads
\newlength{\@tuftex@fullwidth}
\newlength{\@tuftex@caption@fill}

\NewDocumentCommand{\@tuftex@Recalculate}{}{%
  \setlength{\@tuftex@overhang}{\marginparwidth}
  \addtolength{\@tuftex@overhang}{\marginparsep}

  \setlength{\@tuftex@fullwidth}{\textwidth}
  \addtolength{\@tuftex@fullwidth}{\marginparsep}
  \addtolength{\@tuftex@fullwidth}{\marginparwidth}

  \setlength{\@tuftex@caption@fill}{\textwidth}
  \addtolength{\@tuftex@caption@fill}{\marginparsep}
}

\AtBeginDocument{\@tuftex@Recalculate}

% from original tufte-common.def
% Full-page-width area
%
%\newenvironment{fullwidth}
%  {\ifthenelse{\boolean{@tufte@symmetric}}%
%     {\ifthenelse{\boolean{@tufte@changepage}}{\begin{adjustwidth*}{}{-\@tufte@overhang}}{\begin{adjustwidth}[]{}{-\@tufte@overhang}}}%
%     {\begin{adjustwidth}{}{-\@tufte@overhang}}%
%  }%
%  {\ifthenelse{\boolean{@tufte@symmetric}}%
%    {\ifthenelse{\boolean{@tufte@changepage}}{\end{adjustwidth*}}{\end{adjustwidth}}}%
%    {\end{adjustwidth}}%
%  }

\NewDocumentEnvironment{fullwidth}{}%
{\begin{adjustwidth}{}{-\@tuftex@overhang}}%
{\end{adjustwidth}}