Add xeCJKVerbAddon to default fvset config for Chinese documents

Prevent xeCJK from adding extra spaces in fancyvrb Verbatim enviroment.

Fixes: #9414

Signed-off-by: Shengjing Zhu <i@zhsj.me>

doc/latex.rst

@@ -531,6 +531,10 @@ Keys that don't need to be overridden unless in special cases are:
       Changed default for ``'pdflatex'``. Previously it was using 
       ``'\\fvset{fontsize=\\small}'``.
 
+   .. versionchanged:: 4.1.0
+      Changed default for Chinese documents to
+      ``'\\fvset{fontsize=\\small,formatcom=\\xeCJKVerbAddon}'``
+
 Keys that are set by other options and therefore should not be overridden are:
 
 ``'docclass'``

sphinx/builders/latex/constants.py

@@ -198,6 +198,9 @@ ADDITIONAL_SETTINGS: Dict[Any, Dict[str, Any]] = {
         'polyglossia':  '',
         'babel':        '\\usepackage{babel}',
         'fontenc':      '\\usepackage{xeCJK}',
+        # set formatcom=\xeCJKVerbAddon to prevent xeCJK from adding extra spaces in
+        # fancyvrb Verbatim enviroment.
+        'fvset':        '\\fvset{fontsize=\\small,formatcom=\\xeCJKVerbAddon}',
     },
     ('xelatex', 'el'): {
         'fontpkg':      XELATEX_GREEK_DEFAULT_FONTPKG,