ENH: Don't fragment manager if convert is no-op (#53977)

Author: phofl

pandas/core/internals/blocks.py

@@ -480,7 +480,15 @@ def convert(
             return [self.copy()] if copy else [self]
 
         if self.ndim != 1 and self.shape[0] != 1:
-            return self.split_and_operate(Block.convert, copy=copy, using_cow=using_cow)
+            blocks = self.split_and_operate(
+                Block.convert, copy=copy, using_cow=using_cow
+            )
+            if all(blk.dtype.kind == "O" for blk in blocks):
+                # Avoid fragmenting the block if convert is a no-op
+                if using_cow:
+                    return [self.copy(deep=False)]
+                return [self.copy()] if copy else [self]
+            return blocks
 
         values = self.values
         if values.ndim == 2:

pandas/tests/frame/methods/test_replace.py

@@ -1592,3 +1592,10 @@ def test_replace_categorical_no_replacement(self):
 
         result = df.replace(to_replace=[".", "def"], value=["_", None])
         tm.assert_frame_equal(result, expected)
+
+    def test_replace_object_splitting(self):
+        # GH#53977
+        df = DataFrame({"a": ["a"], "b": "b"})
+        assert len(df._mgr.blocks) == 1
+        df.replace(to_replace=r"^\s*$", value="", inplace=True, regex=True)
+        assert len(df._mgr.blocks) == 1