Disable default kerning when letterspacing is active

[peaR-red]

This PR adds a check to fonts.readers.harf to bypass GPOS kerning when tracking is active when using HarfBuzz.

[FrankMittelbach]

What exactly is the rationale here? I don't quite see why you want to disable kerning just because you do some light tracking/letter spacing. For example, suppose you have "AV" then even with minimal letter spacing you suddenly get "A V".

[peaR-red]

Consider the MWE, compiled with lualatex:

\documentclass{article}
\usepackage{fontspec}
\setmainfont{lmroman10-regular}

\begin{document}
\setlength{\fboxrule}{0pt}
\newcommand\test[1]{\noindent{\framebox[6em][l]{\ttfamily#1:}} {\addfontfeature{LetterSpace=50, Renderer=#1} DOCUMENTATION}\par}


\test{Base}
\test{Node}
\test{Harfbuzz}
\test{OpenType}
\end{document}

Note that with OpenType and HarfBuzz the "NT" does not comply with LetterSpace.

This effect is not limited to Latin Modern; it is also visible in other fonts.

[Udi-Fogiel]

The difference you see is because of discretionary nodes (harf and node insert different break points). You can get the same thing with mode=node if you manually insert them

\documentclass{article}
\usepackage{fontspec}
\setmainfont{lmroman10-regular}

\begin{document}
\setlength{\fboxrule}{0pt}
\newcommand\test[1]{\hbox{\addfontfeature{LetterSpace=50, Renderer=#1} DOCUMEN\discretionary{-}{-}{T}\discretionary{}{}{}ATION}}

\test{Node}
\end{document}

AFAICT your patch also affect kerning/spacing for harf, which I'm not sure is desirable.

I think the real problem is either discretionary nodes are not handled well in luaotfload-letterspace.lua, or maybe harf should not insert consecutive disc nodes (although I'm not sure if the latter is correct behavior or not).