BUG: fix online ewma with CoW (#55735)

Author: jorisvandenbossche

pandas/core/window/ewm.py

@@ -1068,7 +1068,7 @@ def mean(self, *args, update=None, update_times=None, **kwargs):
                 result_kwargs["columns"] = self._selected_obj.columns
             else:
                 result_kwargs["name"] = self._selected_obj.name
-            np_array = self._selected_obj.astype(np.float64).to_numpy()
+            np_array = self._selected_obj.astype(np.float64, copy=False).to_numpy()
         ewma_func = generate_online_numba_ewma_func(
             **get_jit_arguments(self.engine_kwargs)
         )

pandas/core/window/online.py

@@ -52,7 +52,7 @@ def online_ewma(
         exponentially weighted mean accounting minimum periods.
         """
         result = np.empty(values.shape)
-        weighted_avg = values[0]
+        weighted_avg = values[0].copy()
         nobs = (~np.isnan(weighted_avg)).astype(np.int64)
         result[0] = np.where(nobs >= minimum_periods, weighted_avg, np.nan)