Switch TeX baseline detection to use a dvi special.

Insert a `\special` at the end of the TeX output and record the baseline
position there.  This should be more robust than trying to guess the
format of the dvi preamble, and more importantly this will also allow
detecting the last baseline of *multiline* TeX output, which will then
allow letting TeX handle multiline strings itself (while keeping the
correct alignment).

Author: anntzer

lib/matplotlib/dviread.py

@@ -291,40 +291,11 @@ def _read(self):
         Read one page from the file. Return True if successful,
         False if there were no more pages.
         """
-        # Pages appear to start with the sequence
-        #   bop (begin of page)
-        #   xxx comment
-        #   <push, ..., pop>  # if using chemformula
-        #   down
-        #   push
-        #     down
-        #     <push, push, xxx, right, xxx, pop, pop>  # if using xcolor
-        #     down
-        #     push
-        #       down (possibly multiple)
-        #       push  <=  here, v is the baseline position.
-        #         etc.
-        # (dviasm is useful to explore this structure.)
-        # Thus, we use the vertical position at the first time the stack depth
-        # reaches 3, while at least three "downs" have been executed (excluding
-        # those popped out (corresponding to the chemformula preamble)), as the
-        # baseline (the "down" count is necessary to handle xcolor).
-        down_stack = [0]
         self._baseline_v = None
         while True:
             byte = self.file.read(1)[0]
             self._dtable[byte](self, byte)
             name = self._dtable[byte].__name__
-            if name == "_push":
-                down_stack.append(down_stack[-1])
-            elif name == "_pop":
-                down_stack.pop()
-            elif name == "_down":
-                down_stack[-1] += 1
-            if (self._baseline_v is None
-                    and len(getattr(self, "stack", [])) == 3
-                    and down_stack[-1] >= 4):
-                self._baseline_v = self.v
             if byte == 140:                         # end of page
                 return True
             if self.state is _dvistate.post_post:   # end of file
@@ -457,6 +428,8 @@ def _fnt_num(self, new_f):
     @_dispatch(min=239, max=242, args=('ulen1',))
     def _xxx(self, datalen):
         special = self.file.read(datalen)
+        if special == b'matplotlibbaselinemarker':
+            self._baseline_v = self.v
         _log.debug(
             'Dvi._xxx: encountered special: %s',
             ''.join([chr(ch) if 32 <= ch < 127 else '<%02x>' % ch

lib/matplotlib/texmanager.py

@@ -217,7 +217,7 @@ def make_tex(self, tex, fontsize):
 %% when using psfrag which gets confused by it.
 \fontsize{%(fontsize)f}{%(baselineskip)f}%%
 \ifdefined\psfrag\else\hbox{}\fi%%
-{%(fontcmd)s %(tex)s}
+{%(fontcmd)s %(tex)s}\special{matplotlibbaselinemarker}
 \end{document}
 """
         Path(texfile).write_text(tex_template % {