Using post_linebreak_filter in LuaTeX to set height and depth of boxes

After a sleepless night I found a solution. In the append_to_vlist_filter you can manipulate normal lines (n.subtype==1), equations (n.subtype==6, that means lines of display math) und equationnumbers (n.subtype==7, contents after eqno or leqno). Now every display math line has a corrected height and depth, too. This is grid typesetting!!!! :)

Complete MWE:

documentclass[12pt]{article}

usepackage{showframe,color,unicode-math}
usepackage{lua-visual-debug}

% Check grid typesetting
defgrid{%
    vtop to0pt{%
        hsize=0pt noindent color{red}%
        XXXXXXXXXXXXXXXXXXvss}}

raggedbottom

directlua{
function fixlines(h,c)
  local B=tex.baselineskip.width
  for n in node.traverse(h) do
  %
  % correct height and depth
   if n.id==0 and (n.subtype==1 or n.subtype==6 or n.subtype==7) then
    n.height = (math.ceil((n.height+.29*B)/B)-.3)*B
    n.depth  = (math.ceil((n.depth+.69*B)/B)-.7)*B
   end
%
% lineskip or baselineskip: kill glue
   if n.id==12 and (n.subtype==1 or n.subtype==2) then
    n.width=0 % kill glue
   end
  end
 return h
end
luatexbase.add_to_callback('append_to_vlist_filter', fixlines, 'fix line spacing')
}

% test big objects
def?{begin{pmatrix}1&0&0&1&0&0&1end{pmatrix}}

begin{document}

parskip0pt
lineskip0pt
lineskiplimit0pt
normallineskiplimit0pt
jot0pt
% displaymath spacing
abovedisplayskipbaselineskip
abovedisplayshortskipbaselineskip
belowdisplayskipbaselineskip
belowdisplayshortskipbaselineskip
    
noindent grid I like grid typesetting. Let us test some inline math. Let us
test some very big inline math with : $?₃ = ?displaystyle∫$. The next line obeys the grid, very good!
    
What about detokenize{$$}-math?%vrule depth.3baselineskip width2pt
% Sorry for using $$, but I usually use Plain LuaTeX.
$$
X+A+a+b+c+d∬~hbox{Now I have the same baseline as the red X!}
$$
The grid is obeyed after all diplay math lines. Their height and depth do not
matter any more!
    
end{document}

Edit: The solution above suffers from gobbled baselineskip in some situations (see wipet's comment). I found out that you have to calculate the interline skips on your own when using append_to_vlist_filter, otherwise they are gobbled.

The calculation is taken from: Underline part of a word while preserving kerning

So replace the luacode in the MWE by:

directlua{
function correctheight(h, loc)
  local B=tex.baselineskip.width
  for n in node.traverse(h) do
  % correct height and depth of lines (post_linebreak) and display math incl.
  % (l)eqno
    if loc=='post_linebreak' or loc=='equation' or loc=='equation_number' then
    n.height = (math.ceil((n.height+.29*B)/B)-.3)*B
    n.depth  = (math.ceil((n.depth+.69*B)/B)-.7)*B
   end
  end
 return h
end
%
function makegrid(h, loc, prev, mirror)
 correctheight(h, loc)
% If append_to_vlist_filter is used then you have to calculate interline skips
% on your own, otherwise they are gobbled! 
 local new_prev = mirror and h.height or h.depth
  if prev > -65536000 then
   local lineglue = tex.baselineskip.width - prev - (mirror and h.depth or h.height)
   local skip
    if lineglue < tex.lineskiplimit then
     skip = node.new('glue', 1)
     node.setglue(skip, node.getglue(tex.lineskip))
    else
     skip = node.new('glue', 2)
     node.setglue(skip, node.getglue(tex.baselineskip))
     skip.width = lineglue
    end
   skip.next = h
   h = skip
  end
 return h, new_prev
end
luatexbase.add_to_callback('append_to_vlist_filter', makegrid, 'make grid')}