Fix test logic and comments per review

Author: pareshjoshij

Lib/test/test_email/test__header_value_parser.py

@@ -3257,14 +3257,11 @@ def test_long_filename_attachment(self):
 
     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
+        # the previous line's trailing whitespace should not trigger a double newline.
         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"
-
+        # The non-whitespace text needs to exactly fill the max_line_length (10).
+        text = ("a" * 9) + ", " + ("b" * 20)
+        expected = ("a" * 9) + ",\n " + ("b" * 20) + "\n"
         token = parser.get_address_list(text)[0]
         self._test(token, expected, policy=policy)
 

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

@@ -1 +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.
+Fix a bug in the :mod:`email.policy.default` folding algorithm which incorrectly resulted in a doubled newline when a line ending at exactly max_line_length was followed by an unfoldable token.