Add support for \addvspace

[claell]

Continuation from #125 to not make that issue too long and keep discussions on different topics easier to follow.

I think in HTML, this CSS definition should suppress additional spaces after multiple \epubvspace commands that immediately follows each other:

\Css{.vspace + .vspace{height:0;margin:0;}}

Originally posted by @michal-h21 in #125 (comment)

Thanks! I had a similar idea after thinking about it again.

Unfortunately, after testing, it doesn't work as expected.
I also created a MWE, as this seems to be needed. 🙂

LaTeX:

\documentclass{article}
\usepackage{addvspacetest}
\usepackage{lipsum}

\begin{document}

\section{hello}
\lipsum[1-2]

\epubaddvspace{2em}
third: \lipsum[3]

\epubaddvspace{2em}
\epubaddvspace{2em}
fourth: \lipsum[4]
\end{document}

Style:

\ProvidesPackage{addvspacetest}

\newcommand\epubaddvspace[1]{\addvspace{#1}}

.4ht file:

\def\pt:to:em#1{\strip@pt\dimexpr#1/\f@size em}
\renewcommand\epubaddvspace[1]{\edef\:vspacesize{\pt:to:em{#1}}\a:epubaddvspace}
\NewConfigure{epubaddvspace}{1}
\Configure{epubaddvspace}{\ifvmode\IgnorePar\fi\EndP\HCode{<div class="addvspace" style="height:\:vspacesize">}\:nbsp\HCode{</div>}}
\Css{.addvspace + p{margin-top:0pt;}}
\Css{p + .addvspace{margin-top:0pt;}}
\Css{.addvspace + .addvspace{height:0;margin:0;}}

Command to invoke tex4ebook

tex4ebook -f epub3 test.tex p-indent

Result PDF:

Result EPUB:

Also, as general note: This won't work as \addvspace, which would use the biggest space, and not the first one (as this example does IMHO). For me, that is not a problem, as subsequent spaces would have the same height (at least currently). But just as a note.

[michal-h21]

I've realized that there is a bug in \Configure{epubaddvspace}. It didn't start a paragraph. Here is a fixed version:

\def\pt:to:em#1{\strip@pt\dimexpr#1/\f@size em}
\renewcommand\epubaddvspace[1]{\edef\:vspacesize{\pt:to:em{#1}}\a:epubaddvspace}
\NewConfigure{epubaddvspace}{1}
\Configure{epubaddvspace}{\ifvmode\IgnorePar\fi\EndP\HCode{<div class="addvspace" style="height:\:vspacesize">}\HCode{</div>}\par}
\Css{.addvspace + p{margin-top:0pt;}}
\Css{p + .addvspace{margin-top:0pt;}}
\Css{.addvspace + .addvspace{height:0;margin:0;}}

Regarding the issue with multiple spaces, I forgot that we set explicitly the height using inline styles. This overwrites anything that is written in the CSS file. In this case, I think it is best to use a Lua build file that will remove extra spaces:

local domfilter = require "make4ht-domfilter"

local process = domfilter {
  function(dom)
    for _, vspace in ipairs(dom:query_selector(".addvspace + .addvspace")) do
      vspace:remove_node()
    end
    return dom
  end
}

Make:match("html$", process)

Compile using

 $ tex4ebook -e build.lua -f epub3 test.tex p-indent

[claell]

What is the effect of (not) starting a paragraph in the command?

I also noticed that you removed the non-breaking space. I already wondered whether it should be there, before.

With the additional Lua build file, it works for me, now!

[michal-h21]

It caused the build file to work incorrectly, because it detected two vspaces as following each other even if there was a text between them. Paragraph element is needed to prevent that.