TST: Add test with mask on LHS for test_setitem_with_unaligned_tz_aware_datetime_column

GH12981: addressed jreback's comment

Author: ajenkins-cargometrics

doc/source/whatsnew/v0.18.1.txt

@@ -434,5 +434,4 @@ Bug Fixes
 - ``pd.read_excel()`` now accepts path objects (e.g. ``pathlib.Path``, ``py.path.local``) for the file path, in line with other ``read_*`` functions (:issue:`12655`)
 - ``pd.read_excel()`` now accepts column names associated with keyword argument ``names``(:issue `12870`)
 
-- Bug in ``DataFrame._santize_column`` now preserves Timezone when RHS is a Series (:issue `12981`)
-
+- Bug in ``DataFrame`` timezone lost when assigning tz-aware datetime ``Series`` with alignment (:issue `12981`)

pandas/tests/frame/test_indexing.py

@@ -1950,6 +1950,9 @@ def test_setitem_with_unaligned_tz_aware_datetime_column(self):
         df['dates'] = column[[1, 0, 2]]
         assert_series_equal(df['dates'], column)
 
+        df.loc[[0, 1, 2], 'dates'] = column[[1, 0, 2]]
+        assert_series_equal(df['dates'], column)
+
     def test_setitem_datetime_coercion(self):
         # GH 1048
         df = pd.DataFrame({'c': [pd.Timestamp('2010-10-01')] * 3})