"Actually" plain beamer's frame with (left) sidebar themes: problem with footnotes' vertical position

Question

I'm in the process of creating a customized beamer's frame "actually" plain with a sidebar theme (PaloAlto).

By:

• setting the frametitle's width to \paperwidth instead of \paperwidth-\beamer@sidebarwidth,
• making use of \newgeometry,

I almost managed (for "right" PaloAlto as well), except for the distance between the last footnotes' line and the bottom of the page: it is wider than for ordinary (plain) frames, as shown by the following MCE.

Of course, I can play with the foot option in the new geometry but I can't figure out what's the precise dimension involved.

Hence the question: how to make, in this customized beamer's plain frame, the distance between the last footnotes' line and the bottom of the page the same as for ordinary (plain) frames?

\documentclass[t]{beamer}
\usepackage{xifthen}
\usepackage{multido}

\geometry{verbose}

\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
      }%
    }%
    % Customize the geometry (changes from options of geometry package in
    % `beamer.cls' flagged with `% <- !')
    \newgeometry{%
      % hmargin=1cm,% <- !
      vmargin=0cm,%
      head=0.5cm,%
      headsep=0pt,%
      foot=0.5cm,%
      textwidth=\textwidth+\beamer@sidebarwidth,% <- !
      includeheadfoot,% <- !
    }%
  }{%
  }%
  %
  \begin{frame}[plain,#1]%
  }{%
  \end{frame}%
  \restoregeometry
}
\makeatother

\usetheme{PaloAlto}
% \usetheme[right]{PaloAlto}

\usecolortheme{seahorse}

\newcommand{\foo}[1]{%
  \multido{\i=1+1}{#1}{%
    \i~Foo
  }%
}

\begin{document}
\begin{frame}[plain]
  \frametitle{Default plain frame \alert{before} customization}
  \foo{25}\footnote{\foo{25}}
\end{frame}
%
\begin{frame}
  \frametitle{Default frame \alert{before} customization}
  \foo{25}\footnote{\foo{25}}
\end{frame}
%
\begin{myplainframe}
  \frametitle{Customized plain frame}
  \foo{25}\footnote{\foo{25}}
\end{myplainframe}
%
\begin{frame}[plain]
  \frametitle{Default plain frame \alert{after} customization}
  \foo{25}\footnote{\foo{25}}
\end{frame}
%
\begin{frame}
  \frametitle{Default frame \alert{after} customization}
  \foo{25}\footnote{\foo{25}}
\end{frame}
\end{document}

Answer 1

Combining https://tex.stackexchange.com/a/198224/36296 and https://tex.stackexchange.com/a/113412/36296 you could simply do:

\documentclass[t]{beamer}
\usepackage{xifthen}
\usepackage{multido}

\usetheme{PaloAlto}
% \usetheme[right]{PaloAlto}
 \setbeamercolor{background canvas}{bg=yellow!50}

\usecolortheme{seahorse}

\newcommand{\foo}[1]{%
  \multido{\i=1+1}{#1}{%
    \i~Foo
  }%
}

\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,#1]%
  }{%
  \end{frame}%

}
\makeatother

\begin{document}

\begin{myplainframe}
  \frametitle{Customized plain frame}
  \foo{25}\footnote{\foo{25}}
\end{myplainframe}

\end{document}