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Author: angela-tarantula

doc/source/whatsnew/v3.0.0.rst

@@ -1145,6 +1145,7 @@ Reshaping
 ^^^^^^^^^
 - Bug in :func:`qcut` where values at the quantile boundaries could be incorrectly assigned (:issue:`59355`)
 - Bug in :meth:`DataFrame.combine_first` not preserving the column order (:issue:`60427`)
+- Bug in :meth:`DataFrame.combine_first` where large ``int64``/``uint64`` values could lose precision when an outer alignment introduced missing values. (:issue:`60128`)
 - Bug in :meth:`DataFrame.explode` producing incorrect result for :class:`pyarrow.large_list` type (:issue:`61091`)
 - Bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
 - Bug in :meth:`DataFrame.join` when a :class:`DataFrame` with a :class:`MultiIndex` would raise an ``AssertionError`` when :attr:`MultiIndex.names` contained ``None``. (:issue:`58721`)

pandas/core/frame.py

@@ -9174,7 +9174,7 @@ def _cast_large_numpy_ints_to_nullable(df: DataFrame) -> DataFrame:
 
                 return df.astype(cast_map) if cast_map else df
 
-            # Cast any side that will gain rows on outer align (introduces <NA>).
+            # Only need to cast sides that gain rows on outer align (introduces <NA>).
             if len(other.index.difference(self.index, sort=False)):
                 self = _cast_large_numpy_ints_to_nullable(self)
             if len(self.index.difference(other.index, sort=False)):