Abbr should respect AtomicStrings

Fixes Python-Markdown#1512

Author: facelessuser

docs/changelog.md

@@ -25,6 +25,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `md_in_html` handle tags within inline code blocks better (#1075).
 * `md_in_html` fix handling of one-liner block HTML handling (#1074).
 * Ensure `<center>` is treated like a block-level element (#1481).
+* Ensure that `abbr` extension respects `AtomicString` and does not process
+  perceived abbreviations in these strings (#1512).
 
 ## [3.7] -- 2024-08-16
 

markdown/extensions/abbr.py

@@ -98,20 +98,22 @@ def iter_element(self, el: etree.Element, parent: etree.Element | None = None) -
         for child in reversed(el):
             self.iter_element(child, el)
         if text := el.text:
-            for m in reversed(list(self.RE.finditer(text))):
-                if self.abbrs[m.group(0)]:
-                    abbr = self.create_element(self.abbrs[m.group(0)], m.group(0), text[m.end():])
-                    el.insert(0, abbr)
-                    text = text[:m.start()]
-            el.text = text
+            if not isinstance(text, AtomicString):
+                for m in reversed(list(self.RE.finditer(text))):
+                    if self.abbrs[m.group(0)]:
+                        abbr = self.create_element(self.abbrs[m.group(0)], m.group(0), text[m.end():])
+                        el.insert(0, abbr)
+                        text = text[:m.start()]
+                el.text = text
         if parent is not None and el.tail:
             tail = el.tail
             index = list(parent).index(el) + 1
-            for m in reversed(list(self.RE.finditer(tail))):
-                abbr = self.create_element(self.abbrs[m.group(0)], m.group(0), tail[m.end():])
-                parent.insert(index, abbr)
-                tail = tail[:m.start()]
-            el.tail = tail
+            if not isinstance(tail, AtomicString):
+                for m in reversed(list(self.RE.finditer(tail))):
+                    abbr = self.create_element(self.abbrs[m.group(0)], m.group(0), tail[m.end():])
+                    parent.insert(index, abbr)
+                    tail = tail[:m.start()]
+                el.tail = tail
 
     def run(self, root: etree.Element) -> etree.Element | None:
         ''' Step through tree to find known abbreviations. '''

tests/test_syntax/extensions/test_abbr.py

@@ -30,6 +30,18 @@ class TestAbbr(TestCase):
 
     default_kwargs = {'extensions': ['abbr']}
 
+    def test_ignore_atomic(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                This <https://example.com/{YAFR}>
+
+                *[YAFR]: Yet Another Feature Request
+                """
+            ),
+            '<p>This <a href="https://example.com/{YAFR}">https://example.com/{YAFR}</a></p>'
+        )
+
     def test_abbr_upper(self):
         self.assertMarkdownRenders(
             self.dedent(