fix: add missing newline after inline table in serialization

When an inline table without a trailing newline was followed by a new
key-value pair, the serialization would concatenate them on the same
line (e.g. 'a.b = {}c = 3' instead of 'a.b = {}\nc = 3').

This ensures a newline is inserted before rendering a simple item
when the preceding content in a table doesn't end with one.

Fixes #440

Author: daniel7an

tests/test_toml_document.py

@@ -1314,3 +1314,25 @@ def test_appending_to_super_table():
 """
 
     assert doc.as_string() == expected
+
+
+def test_missing_newline_after_inline_table_issue_440():
+    """Regression test for https://github.com/python-poetry/tomlkit/issues/440"""
+    content = """[x]\na.b = {}"""
+    doc = parse(content)
+    doc["x"]["c"] = 3
+    expected = "[x]\na.b = {}\nc = 3\n"
+    assert doc.as_string() == expected
+
+    # With trailing newline should also work
+    content2 = """[x]\na.b = {}\n"""
+    doc2 = parse(content2)
+    doc2["x"]["c"] = 3
+    assert doc2.as_string() == expected
+
+    # Non-dotted key variant
+    content3 = """[x]\na = {}"""
+    doc3 = parse(content3)
+    doc3["x"]["c"] = 3
+    expected3 = "[x]\na = {}\nc = 3\n"
+    assert doc3.as_string() == expected3

tomlkit/container.py

@@ -606,6 +606,8 @@ def _render_table(self, key: Key, table: Table, prefix: str | None = None) -> st
                     cur += "\n"
                 cur += self._render_aot(k, v, prefix=_key)
             else:
+                if k is not None and cur and not cur.endswith("\n"):
+                    cur += "\n"
                 cur += self._render_simple_item(
                     k, v, prefix=_key if key.is_dotted() else None
                 )