fix(inline): use original run length in emphasis multiple-of-3 rule

CommonMark rules 9 and 10 require the multiple-of-3 delimiter test to use
the lengths of the original delimiter runs, not the lengths remaining after
a run has been partially consumed. mistune mutated _Delimiter.length as it
consumed markers and then reused that mutated value in the mod-3 test, so the
leftover of a partially consumed run (e.g. the ** left after *a* closes in
'*a***a*') could no longer match a later delimiter.

Track the original run length on _Delimiter and use it in
_can_match_emphasis_delimiters. Fixes cases such as '*a***a*' and
'*foo***bar*', which two CommonMark reference implementations
(markdown-it-py, commonmark.py) render as <em>a</em>*<em>a</em> and
<em>foo</em>*<em>bar</em>.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: greymoth-jp

src/mistune/inline_parser.py

@@ -529,6 +529,9 @@ class _Delimiter:
     length: int
     can_open: bool
     can_close: bool
+    #: original run length, used by the CommonMark multiple-of-3 rule even
+    #: after the run has been partially consumed
+    orig_length: int = 0
 
 
 def _finalize_emphasis_tokens(tokens: List[Dict[str, Any]], enabled: bool) -> List[Dict[str, Any]]:
@@ -618,7 +621,7 @@ def _split_text_token(
         index = len(parts)
         parts.append({"type": "text", "raw": text[pos:end]})
         if can_open or can_close:
-            delimiters.append(_Delimiter(index, marker, length, can_open, can_close))
+            delimiters.append(_Delimiter(index, marker, length, can_open, can_close, length))
         pos = end
 
 
@@ -733,7 +736,12 @@ def _has_emphasis_content(parts: List[Dict[str, Any]], start: int, end: int) ->
 
 def _can_match_emphasis_delimiters(opener: _Delimiter, closer: _Delimiter) -> bool:
     if opener.can_close or closer.can_open:
-        return (opener.length + closer.length) % 3 != 0 or opener.length % 3 == 0 and closer.length % 3 == 0
+        # CommonMark rules 9 and 10: the multiple-of-3 test uses the lengths of
+        # the original delimiter runs, not the lengths remaining after the runs
+        # have been partially consumed.
+        open_len = opener.orig_length
+        close_len = closer.orig_length
+        return (open_len + close_len) % 3 != 0 or open_len % 3 == 0 and close_len % 3 == 0
     return True
 
 

tests/fixtures/fix-commonmark.txt

@@ -54,6 +54,21 @@ CommonMark delimiter handling is the default.
 <p>*a *<em>b c</em> d</p>
 ````````````````````````````````
 
+The multiple-of-3 rule uses the original delimiter run length, so the
+remainder of a partially consumed run can still match a later delimiter.
+
+```````````````````````````````` example
+*a***a*
+.
+<p><em>a</em>*<em>a</em></p>
+````````````````````````````````
+
+```````````````````````````````` example
+*foo***bar*
+.
+<p><em>foo</em>*<em>bar</em></p>
+````````````````````````````````
+
 
 ## Max depth