[luatex] positioning math accents over text fonts

[Ulrike Fischer]

On tex.sx there is a question about using a special text font as
math italic and the positioning of math accents in this case.
https://tex.stackexchange.com/questions/401653/fontspec-unicode-math-mixing-fonts-math-accents

Reading the luatex manual I thought that one need a top_accent field
and value for the relevant chars and so I tried to patch a text font
to add it and got a curious results:

Adding the top_accent field seems to have no effect at all for text
fonts (Arial in my example) but it has an effect if the "text font"
is a math font (Stix Two Math). 

Has someone an idea why the accent is ignoring top_accent in the
first case?

The example is a latex example but if needed I could try to convert
it to plain. I tried with the default font loader and also with a
rather new version of the generic font loader from context which I
created myself (fontloader-reference-2017-10-11.lua). 

\documentclass{article}

\usepackage{luacode}

\begin{luacode}

local patch_arial = function (fontdata)
 if fontdata.psname == "ArialMT"
 then
   fontdata.characters[108]["top_accent"]=300380
  -- fontdata.characters[108]["width"]=3300380 -- to test patch
 end
end

local patch_stix = function (fontdata)
 if fontdata.psname == "STIXTwoMath"
 then
  fontdata.characters[108]["top_accent"]=300380
 end
end


luatexbase.add_to_callback
 (
  "luaotfload.patch_font",
  patch_arial,
  "change_arial"
 )

luatexbase.add_to_callback
 (
  "luaotfload.patch_font",
  patch_stix,
  "change_stix"
 )

\end{luacode}

\usepackage{unicode-math}

\setmainfont{STIXTwoMath}  %displaces accents
%\setmainfont{Arial}       %doesn't work

\setmathfont{Cambria Math}



\begin{document}
 $\rm a $ %load text font in math context once for init
 
 $\Umathaccent fixed 7 \symoperators "00302 {\rm l}$

 $\hat{\rm l}$

 $\Umathaccent fixed 7 \symoperators "00302 {\rm l}$

 $\hat{\rm l}$

\end{document}




-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/