gh-142006: Fix HeaderWriteError in email.policy.default caused by extra newline

Lib/email/_header_value_parser.py

@@ -2792,6 +2792,9 @@ def _steal_trailing_WSP_if_exists(lines):
     if lines and lines[-1] and lines[-1][-1] in WSP:
         wsp = lines[-1][-1]
         lines[-1] = lines[-1][:-1]
+        # gh-142006: if the line is now empty, remove it entirely.
+        if not lines[-1]:
+            lines.pop()
     return wsp
 
 def _refold_parse_tree(parse_tree, *, policy):

Lib/test/test_email/test__header_value_parser.py

@@ -3255,5 +3255,18 @@ def test_long_filename_attachment(self):
             " filename*1*=_TEST_TES.txt\n",
             )
 
+    def test_fold_unfoldable_element_stealing_whitespace(self):
+        # gh-142006: When an element is too long to fit on the current line
+        # the previous line's trailing whitespace
+        policy = self.policy.clone(max_line_length=10)
+
+        # "a," fits. The space after it should wrap to the next line.
+        # The "long" part (20 chars) forces the wrap because 20 > 10.
+        text = "a, " + ("b" * 20)
+        expected = "a,\n " + ("b" * 20) + "\n"
+
+        token = parser.get_address_list(text)[0]
+        self._test(token, expected, policy=policy)
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2025-11-27-10-49-13.gh-issue-142006.nzJDG5.rst

@@ -0,0 +1 @@
+Fix :exc:`email.errors.HeaderWriteError` in :mod:`email.policy.default` which incorrectly added double newlines when folding lines containing unfoldable syntactic elements wider than max_line_len.