BUG: IntervalIndex.unique() only contains the first interval if all interval borders are negative

pandas/core/arrays/interval.py

@@ -1971,10 +1971,10 @@ def _from_combined(self, combined: np.ndarray) -> IntervalArray:
         """
         Create a new IntervalArray with our dtype from a 1D complex128 ndarray.
         """
-        nc = combined.view("i8").reshape(-1, 2)
 
         dtype = self._left.dtype
         if needs_i8_conversion(dtype):
+            nc = combined.view("i8").reshape(-1, 2)
             assert isinstance(self._left, (DatetimeArray, TimedeltaArray))
             new_left: DatetimeArray | TimedeltaArray | np.ndarray = type(
                 self._left
@@ -1985,16 +1985,27 @@ def _from_combined(self, combined: np.ndarray) -> IntervalArray:
             )._from_sequence(nc[:, 1], dtype=dtype)
         else:
             assert isinstance(dtype, np.dtype)
+            nc = np.hstack(
+                [np.real(combined).astype(dtype), np.imag(combined).astype(dtype)]
+            ).reshape(-1, 2)
             new_left = nc[:, 0].view(dtype)
             new_right = nc[:, 1].view(dtype)
         return self._shallow_copy(left=new_left, right=new_right)
 
     def unique(self) -> IntervalArray:
         # No overload variant of "__getitem__" of "ExtensionArray" matches argument
         # type "Tuple[slice, int]"
-        nc = unique(
-            self._combined.view("complex128")[:, 0]  # type: ignore[call-overload]
-        )
+        if needs_i8_conversion(self._left.dtype):
+            nc = unique(
+                self._combined.view("complex128")[:, 0]  # type: ignore[call-overload]
+            )
+        else:
+            nc = unique(
+                # Using .view("complex128") with negatives causes issues.
+                # GH#61917
+                (np.array(self._combined[:, 0], dtype=complex))
+                + (1j * np.array(self._combined[:, 1], dtype=complex))
+            )
         nc = nc[:, None]
         return self._from_combined(nc)
 

pandas/tests/arrays/interval/test_interval.py

@@ -111,6 +111,31 @@ def test_shift_datetime(self):
         with pytest.raises(TypeError, match=msg):
             a.shift(1, fill_value=np.timedelta64("NaT", "ns"))
 
+    def test_unique_with_negatives(self):
+        # GH#61917
+        idx_pos = IntervalIndex.from_tuples(
+            [(3, 4), (3, 4), (2, 3), (2, 3), (1, 2), (1, 2)]
+        )
+        result = idx_pos.unique()
+        expected = IntervalIndex.from_tuples([(3, 4), (2, 3), (1, 2)])
+        tm.assert_index_equal(result, expected)
+
+        idx_neg = IntervalIndex.from_tuples(
+            [(-4, -3), (-4, -3), (-3, -2), (-3, -2), (-2, -1), (-2, -1)]
+        )
+        result = idx_neg.unique()
+        expected = IntervalIndex.from_tuples([(-4, -3), (-3, -2), (-2, -1)])
+        tm.assert_index_equal(result, expected)
+
+        idx_mix = IntervalIndex.from_tuples(
+            [(1, 2), (0, 1), (-1, 0), (-2, -1), (-3, -2), (-3, -2)]
+        )
+        result = idx_mix.unique()
+        expected = IntervalIndex.from_tuples(
+            [(1, 2), (0, 1), (-1, 0), (-2, -1), (-3, -2)]
+        )
+        tm.assert_index_equal(result, expected)
+
 
 class TestSetitem:
     def test_set_na(self, left_right_dtypes):