Make sure to only copy nodes that have the same indentation in diff parser

parso/python/diff.py

@@ -636,8 +636,9 @@ class _NodesTree(object):
         # lines above there was an invalid indent.
         indentation = _get_indentation(tree_nodes[0])
         for i, c in enumerate(tree_nodes):
-            if c.start_pos[1] < indentation:
+            if c.start_pos[1] != indentation:
                 tree_nodes = tree_nodes[:i]
+                break
 
         old_working_stack = list(self._working_stack)
         old_prefix = self.prefix

test/test_diff_parser.py

@@ -1408,7 +1408,7 @@ def test_error_dedent_in_function(differ):
     differ.parse(code2, parsers=ANY, copies=ANY, expect_error_leaves=True)
 
 
-def test_x(differ):
+def test_with_formfeed(differ):
     code1 = dedent('''\
         @bla
         async def foo():
@@ -1427,3 +1427,28 @@ def test_x(differ):
         ''')
     differ.initialize(code1)
     differ.parse(code2, parsers=ANY, copies=ANY, expect_error_leaves=True)
+
+
+def test_x(differ):
+    code1 = dedent('''\
+def foo():
+    return
+
+@bla
+    a
+def foo():
+    a
+    b
+    c
+        ''')
+    code2 = dedent('''\
+def foo():
+    return
+
+@bla
+    a
+    b
+    c
+        ''')
+    differ.initialize(code1)
+    differ.parse(code2, parsers=ANY, copies=ANY, expect_error_leaves=True)