Beamer footline weird interaction with ifnum and frame number

Question

I am creating a theme for a progressbar that shows for all slides except the first slide. I place the progressbar in the footline as I want it to show up at the bottom.

What I don't understand is why this doesn't work (actually it shows a very tiny progressbar if you use zoom in a lot)

\ifnum\insertframenumber>1\relax%
%progress bar code
\fi

and this works

\ifnum\insertframenumber=1\relax%
\else
%progress bar code
\fi

Another issue is that if I dont add \ifnum\inserttotalframenumber>1, it will fail on first run but works in subsequent runs. It seems that latex compiles the code twice in the first run with the first compilation having \inserttotalframenumber equal to 1 (causing division by 0). Is this correct?

I put the code below. There is a comment for working code and not working code. It is compiled with Luatex. I am not sure if pdflatex will work.

% filename: beamerouterthemetest.sty
\mode<presentation>

\RequirePackage{tikz}

\newcount\progressbar@current
\newcount\progressbar@total
\newdimen\progressbar@curwidth
\newdimen\progressbar@width
\newdimen\progressbar@height

\progressbar@width=\paperwidth
\progressbar@height=1mm

% the progress bar
\def\progressbar@progressbar{%
    \progressbar@current=\insertframenumber
    \progressbar@total=\inserttotalframenumber
    \advance\progressbar@current-1
    \advance\progressbar@total-1
    
    \progressbar@curwidth=\progressbar@width
    \multiply\progressbar@curwidth by \progressbar@current
    \divide\progressbar@curwidth by \progressbar@total

  \begin{tikzpicture}[]
      \draw[fill=fg] %
        (0pt, 0pt) rectangle ++ (\progressbar@curwidth, \progressbar@height);
  \end{tikzpicture}%
}

% THIS WORKS!!!!!!!!!!!!!!!!!!!
% Footline
\defbeamertemplate*{footline}{test}[1][] {
  \ifnum\insertframenumber=1\relax%
  \else
    % Added next ifnum to avoid division by 0 crash in first run
    \ifnum\inserttotalframenumber>1\relax%
      \begin{beamercolorbox}[wd=\paperwidth]{progressbar}%
        \progressbar@progressbar%
      \end{beamercolorbox}%
    \fi
  \fi
}

% THIS DOESNT WORK!!!!!!!!!!!!!!!!!!!
% % Footline
% \defbeamertemplate*{footline}{test}[1][] {
%   \ifnum\insertframenumber>1\relax%
%     % Added next ifnum to avoid division by 0 crash in first run
%     \ifnum\inserttotalframenumber>1\relax%
%       \begin{beamercolorbox}[wd=\paperwidth]{progressbar}%
%         \progressbar@progressbar%
%       \end{beamercolorbox}%
%     \fi
%   \fi
% }

\mode<all>

% filename: test-example.tex
\documentclass[aspectratio=169]{beamer}
\useoutertheme{test} 

\begin{document}
\begin{frame}{Slide 1}
\end{frame}
\begin{frame}{Slide 2}
\end{frame}
\begin{frame}{Slide 3}
\end{frame}
\begin{frame}{Slide 4}
\end{frame}
\begin{frame}{Slide 5}
\end{frame}
\begin{frame}{Slide 6}
\end{frame}
\begin{frame}{Slide 7}
\end{frame}

\end{document}

Answer 1

The problem is that beamer calculates the frame geometry, including the height of the footline, at the start of your document. At this place, the frame number will obviously not be larger than 1, so the foot height is calculated to be 0pt and thus your footline is not visible.

You can avoid this problem by making sure that your footline has a suitable height at the start of the document:

% filename: test-example.tex
\documentclass[aspectratio=169]{beamer}

\makeatletter
% filename: beamerouterthemetest.sty
\mode<presentation>

\RequirePackage{tikz}

\newcount\progressbar@current
\newcount\progressbar@total
\newdimen\progressbar@curwidth
\newdimen\progressbar@width
\newdimen\progressbar@height

\progressbar@width=\paperwidth
\progressbar@height=1mm

% the progress bar
\def\progressbar@progressbar{%
    \progressbar@current=\insertframenumber
    \progressbar@total=\inserttotalframenumber
    \advance\progressbar@current-1
    \advance\progressbar@total-1
    
    \progressbar@curwidth=\progressbar@width
    \multiply\progressbar@curwidth by \progressbar@current
    \divide\progressbar@curwidth by \progressbar@total

  \begin{tikzpicture}[]
      \draw[fill=fg] %
        (0pt, 0pt) rectangle ++ (\progressbar@curwidth, \progressbar@height);
  \end{tikzpicture}%
}

%% THIS WORKS!!!!!!!!!!!!!!!!!!!
%% Footline
%\defbeamertemplate*{footline}{test}[1][] {
%  \ifnum\insertframenumber=1\relax%
%  \else
%    % Added next ifnum to avoid division by 0 crash in first run
%    \ifnum\inserttotalframenumber>1\relax%
%      \begin{beamercolorbox}[wd=\paperwidth]{progressbar}%
%        \progressbar@progressbar%
%      \end{beamercolorbox}%
%    \fi
%  \fi
%}

% THIS DOESNT WORK!!!!!!!!!!!!!!!!!!!
 % Footline
 \defbeamertemplate*{footline}{test}[1][] {
   \ifnum\insertframenumber>1%
     % Added next ifnum to avoid division by 0 crash in first run
     \ifnum\inserttotalframenumber>1\relax%
       \begin{beamercolorbox}[wd=\paperwidth]{progressbar}%
         \progressbar@progressbar%
       \end{beamercolorbox}%
     \fi
   \else
      \vskip0.2cm
   \fi
 }

\mode<all>


\makeatother

\begin{document}
\begin{frame}{Slide 1}
\end{frame}
\begin{frame}{Slide 2}
\end{frame}
\begin{frame}{Slide 3}
\end{frame}
\begin{frame}{Slide 4}
\end{frame}
\begin{frame}{Slide 5}
\end{frame}
\begin{frame}{Slide 6}
\end{frame}
\begin{frame}{Slide 7}
\end{frame}

\end{document}