Restore cached face_areas fast path in calculate_total_face_area

test/grid/grid/test_areas.py

@@ -169,3 +169,30 @@ def test_face_areas_fesom(gridpath):
     uxgrid = ux.open_grid(gridpath("ugrid", "fesom", "fesom.mesh.diag.nc"))
     total_area = uxgrid.calculate_total_face_area()
     nt.assert_almost_equal(total_area, 8.3780, decimal=4)
+
+
+def test_total_face_area_healpix_uses_cached_equal_areas():
+    """Default args must reuse the cached ``face_areas``, not recompute.
+
+    HEALPix faces are exactly equal-area, so the cached sum is exactly 4*pi.
+    Recomputing via quadrature drifts and loses that property.
+    """
+    uxgrid = ux.Grid.from_healpix(zoom=2)
+
+    total_area = uxgrid.calculate_total_face_area()
+
+    nt.assert_allclose(total_area, np.sum(uxgrid.face_areas.values), rtol=0)
+    nt.assert_allclose(total_area, 4 * np.pi, rtol=1e-12)
+
+
+def test_total_face_area_honors_quadrature_kwargs():
+    """Non-default quadrature settings must still trigger a fresh computation."""
+    uxgrid = ux.Grid.from_healpix(zoom=2)
+
+    recomputed = uxgrid.calculate_total_face_area(quadrature_rule="gaussian", order=2)
+
+    nt.assert_allclose(
+        recomputed,
+        np.sum(uxgrid.compute_face_areas(quadrature_rule="gaussian", order=2)),
+        rtol=0,
+    )

uxarray/grid/grid.py

@@ -1992,16 +1992,18 @@ def calculate_total_face_area(
             and not latitude_adjusted_area
         ):
             result = np.sum(self.face_areas.values)
-
-        result = np.sum(
-            self.compute_face_areas(
-                quadrature_rule=quadrature_rule,
-                order=order,
-                latitude_adjusted_area=latitude_adjusted_area,
+        else:
+            result = np.sum(
+                self.compute_face_areas(
+                    quadrature_rule=quadrature_rule,
+                    order=order,
+                    latitude_adjusted_area=latitude_adjusted_area,
+                )
             )
-        )
 
-        RTOL = 1e-6  # 1e-7 had warnings in existing CI tests (as of 2026-08-05), 1e-6 did not.
+        # Choose RTOL. Mostly just an arbitrary decision....
+        # but noting that 1e-9 had warnings in existing CI tests (as of 2026-08-06), while 1e-8 did not.
+        RTOL = 1e-7
         if result > 4 * np.pi * self.sphere_radius**2 * (1 + RTOL):
             warnings.warn(
                 f"Total face area (={result}) exceeds the surface area of the whole sphere "