fix: preserve leading blank lines in MarkdownRenderer

src/mistune/block_parser.py

@@ -109,7 +109,10 @@ def __init__(
 
     def parse_blank_line(self, m: Match[str], state: BlockState) -> int:
         """Parse token for blank lines."""
-        state.append_token({"type": "blank_line"})
+        if state.cursor == 0:
+            state.append_token({"type": "blank_line", "newlines": m.group(0).count("\n")})
+        else:
+            state.append_token({"type": "blank_line"})
         return m.end()
 
     def parse_thematic_break(self, m: Match[str], state: BlockState) -> int:

src/mistune/renderers/markdown.py

@@ -20,10 +20,16 @@ class MarkdownRenderer(BaseRenderer):
     NAME = "markdown"
 
     def __call__(self, tokens: Iterable[Dict[str, Any]], state: BlockState) -> str:
-        out = self.render_tokens(tokens, state)
+        tokens = list(tokens)
+        leading = ""
+        i = 0
+        while i < len(tokens) and tokens[i]["type"] == "blank_line" and "newlines" in tokens[i]:
+            leading += "\n" * tokens[i]["newlines"]
+            i += 1
+        out = self.render_tokens(tokens[i:], state)
         # special handle for line breaks
         out += "\n\n".join(self.render_referrences(state)) + "\n"
-        return strip_end(out)
+        return strip_end(leading + out)
 
     def render_referrences(self, state: BlockState) -> Iterable[str]:
         ref_links = state.env["ref_links"]

tests/test_renderers.py

@@ -144,6 +144,12 @@ def test_prose_not_over_escaped(self):
         ):
             self.assert_round_trip(text)
 
+    def test_leading_blank_lines_preserved(self):
+        # blank_line tokens at the start of the document were rendered as
+        # empty strings, so leading newlines disappeared on reformat
+        self.assertEqual(self.reformat("\nabc"), "\nabc\n")
+        self.assertEqual(self.reformat("\n\n\n\nabc"), "\n\n\n\nabc\n")
+
 
 class TestRendererMethodRegistration(TestCase):
     def test_registered_renderer_method(self):