Preserve array separators when appending after standalone comments

tests/test_items.py

@@ -521,6 +521,26 @@ def test_modify_array_with_comment() -> None:
     assert doc.as_string() == "x = [\n    2\n]"
 
 
+@pytest.mark.parametrize(
+    "content, expected",
+    [
+        ("a = [1\n# end\n]\n", "a = [1,\n# end\n2\n]\n"),
+        ("a = [1,\n# end\n]\n", "a = [1,\n# end\n2,\n]\n"),
+        (
+            "a = [1 # one\n, # separator\n# end\n]\n",
+            "a = [1 # one\n, # separator\n# end\n2\n]\n",
+        ),
+    ],
+)
+def test_append_after_standalone_array_comment(content: str, expected: str) -> None:
+    doc = parse(content)
+    doc["a"].append(2)
+
+    rendered = api.dumps(doc)
+    assert rendered == expected
+    assert parse(rendered)["a"] == [1, 2]
+
+
 def test_append_to_multiline_array_with_comment() -> None:
     doc = parse(
         """\

tomlkit/items.py

@@ -1677,6 +1677,18 @@ def insert(self, pos: int, value: Any) -> None:  # type: ignore[override]
             last_item = self._value[idx - 1]
             if indent is None:
                 indent = last_item.indent
+            # Skip standalone comments when looking for the preceding separator.
+            for previous in self._value[:idx][::-1]:
+                if (
+                    (
+                        previous.value is not None
+                        and not isinstance(previous.value, Null)
+                    )
+                    or previous.comma is not None
+                    or (previous.indent is not None and "," in previous.indent.s)
+                ):
+                    last_item = previous
+                    break
             if not isinstance(last_item.value, Null) and "\n" in default_indent:
                 # Copy the comma from the last item if 1) it contains a value and
                 # 2) the array is multiline