Fix uneven backtick handling

docs/changelog.md

@@ -16,6 +16,8 @@ See the [Contributing Guide](contributing.md) for details.
 
 ### Fixed
 
+* Fix regression in `BacktickInlineProcessor` where text could be lost when opening
+  and closing backticks did not match (#1668).
 * Escape the language name in the `codehilite` extension's non-Pygments output to
   maintain consistency with Pygments output.
 

markdown/inlinepatterns.py

@@ -413,7 +413,7 @@ def __init__(self, pattern: str):
         self.tag = 'code'
         """ The tag of the rendered element. """
 
-    def find_code_spans(self, start: int, text: str) -> tuple[int, int] | None:
+    def find_code_spans(self, start: int, text: str) -> tuple[int, int | None]:
         """Find code spans."""
 
         last = len(text)
@@ -424,10 +424,6 @@ def find_code_spans(self, start: int, text: str) -> tuple[int, int] | None:
             max_ticks += 1
             start += 1
 
-        if not max_ticks:  # pragma: no cover
-            # This is not ever expected to happen.
-            return None
-
         longest_span = 0
         end = 0
 
@@ -453,11 +449,11 @@ def find_code_spans(self, start: int, text: str) -> tuple[int, int] | None:
 
         # Since we didn't find an exact matching start and end,
         # adjust start to match the largest end we could calculate.
-        if longest_span:
+        if 0 < longest_span < max_ticks:
             return start - (max_ticks - longest_span), end - longest_span
 
         # We could not find a suitable pairing
-        return None
+        return start + max_ticks, None
 
     def handleMatch(self, m: re.Match[str], data: str) -> tuple[etree.Element | str | None, int | None, int | None]:
         """
@@ -472,13 +468,13 @@ def handleMatch(self, m: re.Match[str], data: str) -> tuple[etree.Element | str
             return m.group(1).replace('\\\\', self.ESCAPED_BSLASH), m.start(0), m.end(0)
 
         begin = m.start(0)
-        result = self.find_code_spans(begin, data)
-        if result is not None:
-            start, end = result
+        start, end = self.find_code_spans(begin, data)
+        if end is not None:
             el = etree.Element(self.tag)
             el.text = util.AtomicString(util.code_escape(data[start:end].strip()))
-            return el, begin, result[1] + (start - begin)
-        return None, None, None
+            return el, begin, end + (start - begin)
+        # Skip over backtick group as we could find no match
+        return None, begin, start
 
 
 class DoubleTagPattern(SimpleTagPattern):  # pragma: no cover

tests/test_syntax/inline/test_code.py

@@ -79,3 +79,18 @@ def test_noname_tag(self):
                 """
             )
         )
+
+    def test_bigger_end(self):
+
+        # https://github.com/Python-Markdown/markdown/issues/1668
+        self.assertMarkdownRenders(
+            'a ``` b ````` c',
+            '<p>a ``` b ````` c</p>'
+        )
+
+    def test_smaller_end(self):
+
+        self.assertMarkdownRenders(
+            'a ``` b `` c',
+            '<p>a <code>` b</code> c</p>'
+        )