Issuing a full-page figure manually before (not after) the current one

Question

Usually, when there is too much pending floating material (figures, tables and such), LaTeX just gives up on placing it on the current page and generates a “float page” containing only floats before proceeding with the main material. What I would like to do is almost like that, but not quite: to place a single figure on its own page page before the one the current paragraph would go on. I don’t need this figure to be passed through the floats mechanism specifically, but it can if needed.

Unlike a similar question here on TeX.SE, I want the figure emitted on its own page, not retroactively tucked into the bottom of the text on the previous one, so this doesn’t sound impossible in principle from the way TeX operates: the current page has not yet been shipped out and forgotten by TeX, it should be entirely possible to ship out a different one before it. However, I recognize that TeX page breaking may yet decide to break the page before the current line or indeed the current paragraph, so just doing a \shipout from the main document without entering the output routine would be incorrect (even forgetting that it also would not play nice with page headers and such), and I do not understand LaTeX output routine arcana enough to patch it to do this.

(To avoid X/Y issues: I actually want my figure to end up on the left page of a spread and my text on the right, but I don’t think this is expressible in LaTeX without global optimization of pagination.)

If this is relevant, I’m currently using KOMA-Script, scrlayer-scrpage and geometry, but I’m willing to switch if necessary (except maybe for the kaijū that is memoir). M(N)WE:

\documentclass[a4paper]{scrartcl}
\usepackage{geometry}
\usepackage{scrlayer-scrpage}
\usepackage{lipsum}
\title{Hello world} \author{John Doe} \date{}
\begin{document}

% Page 1 and start of page 2 (I want 1 and 3)
\maketitle
\lipsum[1-5]

% Paragraph on page 2 (I want 3) anchoring a big figure
\lipsum*[6][1-3]
\begin{figure}
  % FIXME This ends up on page 3, but I want it on page 2
  \rule{\textwidth}{.8\textheight}
\end{figure}
\lipsum*[6][4-100]

% Some more text that ends up on pages 2 and 4 (I want 3 and 4)
\lipsum[7-10]

\end{document}

Answer 1

In your example document the figure logically fits between between the first two paragraphs and following ones. Why not put it there? --- GOM

% placefigprob.tex  SE 574884

\documentclass[a4paper]{scrartcl}
\usepackage{geometry}
\usepackage{scrlayer-scrpage}
\usepackage{lipsum}
\usepackage{comment}
\usepackage{afterpage}

\newcommand{\showfig}{%
\begin{figure}
   FIXME This ends up on page 3, but I want it on page 2
  \rule{\textwidth}{.8\textheight}
\end{figure}
}
\title{Hello world} \author{John Doe} \date{}
\begin{document}

\maketitle
\textbf{ Page 1 and start of page 2 (I want 1 and 3)} \\
%\maketitle
\lipsum[1-2]
\showfig
\lipsum[3-5]

\textbf{Paragraph on page 2 (I want 3) anchoring a big figure} \\
\lipsum*[6][1-3]
\begin{comment}
\begin{figure}
   FIXME This ends up on page 3, but I want it on page 2
  \rule{\textwidth}{.8\textheight}
\end{figure}
\end{comment}
%\showfig
\lipsum*[6][4-100]

\textbf{ Some more text that ends up on pages 2 and 4 (I want 3 and 4)} \\
\lipsum[7-10]

\end{document}

The above is a semi-modification of your MWE.

Answer 2

Floats are always printed in the order they are created, by type. OTOH, it is possible to slip in a table before a figure, or create a new float type specifically to bypass the queue.

This solution used afterpage, which will always print on the next page (or column). Note that \parindent is not set to 0pt, unlike a figure.

\documentclass[a4paper]{scrartcl}
\usepackage[showframe]{geometry}
\usepackage{scrlayer-scrpage}
\usepackage{afterpage}
\usepackage{lipsum}
\title{Hello world} \author{John Doe} \date{}
\begin{document}
\maketitle
\lipsum[1-5]
\lipsum*[6][1-3]
\afterpage{\vspace*{0pt plus 1fil}\noindent\rule{\textwidth}{.8\textheight}\newpage}% will put \rule on next page
\lipsum*[6][4-100]
\lipsum[7-10]

\end{document}