combine_first's combiner must preserve EA dtypes

Author: angela-tarantula

pandas/core/frame.py

@@ -9026,9 +9026,7 @@ def combine(
         2  NaN  3.0 1.0
         """
 
-        # GH#60128 Prevent lossy conversion of wide integers
-        # by proactively promoting them to their nullable versions
-        # because an outer align will force a round trip through float64.
+        # GH#60128 Prevent lossy conversion of wide integers to float64.
         def _promote_wide_ints(df: DataFrame) -> DataFrame:
             """Promotes int64/uint64 columns to their nullable versions."""
             cast_map: dict[str, str] = {}
@@ -9190,20 +9188,10 @@ def combine_first(self, other: DataFrame) -> DataFrame:
         1  0.0  3.0  1.0
         2  NaN  3.0  1.0
         """
-        from pandas.core.computation import expressions
 
         def combiner(x: Series, y: Series):
-            mask = x.isna()._values
-
-            x_values = x._values
-            y_values = y._values
-
-            # If the column y in other DataFrame is not in first DataFrame,
-            # just return y_values.
-            if y.name not in self.columns:
-                return y_values
-
-            return expressions.where(mask, y_values, x_values)
+            # GH#60128 The combiner must preserve EA dtypes
+            return y if y.name not in self.columns else y.where(x.isna(), x)
 
         if len(other) == 0:
             combined = self.reindex(