myfullwidth.def

%%
% Compute lengths used for full-width displays

\newlength{\overhang}% used by the fullwidth environment and the running heads
\newlength{\fullwidth}
\newlength{\caption@fill}

\newcommand{\TufteRecalculate}{%
  \setlength{\overhang}{\marginparwidth}
  \addtolength{\overhang}{\marginparsep}

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

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

\AtBeginDocument{\TufteRecalculate}





\newcommand{\@float@fullwidth}[3][0pt]{%
  \ifthenelse{\equal{\floatalignment}{b}}%
    {% place caption above figure
      \ifthenelse{\NOT\boolean{@symmetric}\OR\boolean{@float@recto}}%
        {\hfill\smash{\raisebox{#1}{\usebox{#3}}}\par\usebox{#2}\@float@debug{Caption position: [above right]}}% caption on the right
        {\smash{\raisebox{#1}{\usebox{#3}}}\hfill\par\usebox{#2}\@float@debug{Caption position: [above left]}}% caption on the left
    }{% place caption below figure
      \ifthenelse{\NOT\boolean{@symmetric}\OR\boolean{@float@recto}}%
        {\usebox{#2}\par\hfill\smash{\raisebox{#1}{\usebox{#3}}}\@float@debug{Caption position: [below right]}}% caption on the right
        {\usebox{#2}\par\smash{\raisebox{#1}{\usebox{#3}}}\hfill\@float@debug{Caption position: [below left]}}% caption on the left
    }%
}




%%
% Full-page-width area

\newenvironment{fullwidth}
  {\ifthenelse{\boolean{@symmetric}}%
     {\ifthenelse{\boolean{@changepage}}{\begin{adjustwidth*}{}{-\@overhang}}{\begin{adjustwidth}[]{}{-\@overhang}}}%
     {\begin{adjustwidth}{}{-\@overhang}}%
  }%
  {\ifthenelse{\boolean{@symmetric}}%
    {\ifthenelse{\boolean{@changepage}}{\end{adjustwidth*}}{\end{adjustwidth}}}%
    {\end{adjustwidth}}%
  }