Si vous avez un titre des diapositives ou une couleur de fond, c'est plus compliqué :
\documentclass[french,t,11pt,hyperref={colorlinks=true}]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
\usepackage{babel}
\usetheme{PaloAlto}
\author{Sébastien}
\title{Moodle}
\institute{Faculté des Sciences}
\date{\today}
\setbeamertemplate{navigation symbols}{}
\usepackage{xifthen}
\setbeamercolor{background canvas}{bg=yellow!50}
\makeatletter
\newenvironment{myplainframe}[1][]{%
\ifthenelse{\isnamedefined{beamer@sidebarwidth}}{%
% Customize frametitle (changes from \defbeamertemplate*{frametitle}{sidebar
% theme} in `beamerouterthemesidebar.sty' flagged with `% <- !')
\setbeamertemplate{frametitle}
{%
\nointerlineskip%
\ifbeamer@plainframe%
\hbox{\usebeamercolor[bg]{frametitle}%
\@tempdima\paperwidth%\advance\@tempdima-\beamer@sidebarwidth % <- !
\hskip-\beamer@leftmargin\vrule height\beamer@headheight width\@tempdima\hskip-\beamer@rightmargin}%
\nointerlineskip%
\fi%
\vskip-\beamer@headheight%
\vbox to \beamer@headheight{%
\vfil
\leftskip=-\beamer@leftmargin%
\advance\leftskip by0.3cm%
\rightskip=-\beamer@rightmargin%
\advance\rightskip by0.3cm plus1fil%
{\usebeamercolor[fg]{frametitle}\usebeamerfont{frametitle}\insertframetitle\par}%
{\usebeamercolor[fg]{framesubtitle}\usebeamerfont{framesubtitle}\insertframesubtitle\par}%
\vbox{}%
\vskip-1em%
\vfil
}%
}%
\setbeamertemplate{background canvas}
{%
\ifbeamercolorempty[bg]{background canvas}{}{\color{bg}\vrule width\dimexpr\paperwidth+\beamer@leftsidebar\relax height\paperheight}%
}
% Customize the geometry (changes from options of geometry package in
% `beamer.cls' flagged with `% <- !')
\hoffset=-\beamer@leftsidebar
\advance\textwidth\beamer@sidebarwidth
\hsize\textwidth
\columnwidth\textwidth
}{%
}%
%
\begin{frame}[plain,environment=myplainframe,#1]%
}{%
\end{frame}%
}
\makeatother
\begin{document}
\begin{myplainframe}
\frametitle{Really long frame title that stretches the complete width of page}
\titlepage
\end{myplainframe}
\end{document}
![alt text][1]
(modifié de https://tex.stackexchange.com/a/423246/36296 )
depuis [ce fil](https://tex.stackexchange.com/a/423246/36296) sur *stackexchange*)
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-02-06_at_16.40.29_FY5PXOl.png
