Fandol with LuaTeX

Question

The next release of babel will provide basic line breaking for CJK with luatex. When I was making some experiments I discovered an odd and puzzling behaviour in the Fandol family. The file is:

\documentclass{article}

\usepackage{fontspec}
\setmainfont{FandolSong-Regular.otf}

\begin{document}

\section{现代建筑教育}

中国的现代建筑教育体系不是来自本土建筑营造的延续

\end{document}

The result is:

Note the section body follows the title without any vertical break. It works as expected with xetex and with many other fonts in luatex, but not with Fandol. (TeXLive 2018 and 2019.)

Answer 1

What is going on here? Let's start by looking at what \section actually does: In article.cls, \section is defined by

    \newcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\normalfont\Large\bfseries}}

In source2e.pdf, we can find an explenation of all the parameters of \@startsection. For the fifth, it says

afterskip: if positive, then skip to leave below heading, else negative of skip to leave to right of run-in heading.

What you observe with Fandol is a run-in heading with no space between heading and regular text, so obviously the second case is triggered. But this means that the parameter is not positive. This might appear odd, because the command above passes {2.3ex \@plus.2ex}, which certainly looks positive.

But let's take a step back and look what {2.3ex \@plus.2ex} actually is: The \@plus...} is not relevant for us, so we just look at 2.3ex. Here the unit ex is the "x-height" of the current font, but a x-height is not meaningful for a CJK-font like Fandol. Now in the corresponding OpenType field, Fandol has to set some value, so Fandol sets its "x-height" to 0. This is picked up by luaotfload, which passes the x-height 0 to LuaTeX. Now 2.3ex=2.3*"x-height"=2.3*0=0.

So the afterskip parameter is set to 0, which is not positive. So in conformance with it's documentation, \@startsection produces a run-in heading, leaving no space to right of run-in heading.

Now the difference between XeTeX and luaotfload is the handling of missing x-height values:

luaotfload only uses a fallback if the font does not contain any x-height, while XeTeX also uses the fallback if the font explicitly specifies a x-height of 0. You can emulate the XeTeX behaviour in LuaTeX by patching the font:

\documentclass{article}
\directlua{
  luatexbase.add_to_callback("luaotfload.patch_font", function (fontdata)
    local parameters = fontdata.parameters
    if not parameters then return end
    if not (parameters.x_height or parameters[5] or 0) == 0 then return end
    if fontdata.characters and fontdata.characters[120] then
      parameters.x_height = fontdata.characters[120].height
    else
      parameters.x_height = (parameters.ascender or 0)/2
    end
  end, "Fix x-height")
}

\usepackage{fontspec}
\setmainfont{FandolSong-Regular.otf}

\begin{document}
\section{现代建筑教育}

中国的现代建筑教育体系不是来自本土建筑营造的延续

\end{document}

This ensures that the x-height is (almost) never zero, so \section works again: