diff --git a/CHANGELOG.md b/CHANGELOG.md
index a0c25e70..3ee7c95b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,12 +14,13 @@ of being developed elsewhere.
 -----
 
 
-### Version `3.5.X`, first released 2026-10-09
+### Version `3.5.X`, first released XXXXXX
 
 #### Version `3.5.1`, released 2026-??-??
 
 * Update code to prevent deprecation-related warnings
   (https://github.com/NCAS-CMS/cf-plot/issues/72)
+* Prevent striped artefacts on curvilinear longitude/latitude grids such as NEMO.
 
 #### Version `3.5.0`, released 2026-10-09
 
diff --git a/cfplot/blockfill.py b/cfplot/blockfill.py
index 846bcada..1270d7f6 100644
--- a/cfplot/blockfill.py
+++ b/cfplot/blockfill.py
@@ -108,9 +108,11 @@ def _bfill(
                 colarr[pts] = np.size(levels) - 2
 
     if isinstance(field, np.ma.MaskedArray):
-        pts = np.ma.where(field.mask)
-        if np.size(pts) > 0:
-            colarr[pts] = -1
+        # np.ma.where on a scalar False mask raises on newer NumPy.
+        # Use a normalized boolean mask array instead.
+        mask = np.ma.getmaskarray(field)
+        if np.any(mask):
+            colarr[mask] = -1
 
     norm = matplotlib.colors.BoundaryNorm(levels, cmap.N)
 
diff --git a/cfplot/contour.py b/cfplot/contour.py
index f4266017..10f156c1 100644
--- a/cfplot/contour.py
+++ b/cfplot/contour.py
@@ -682,7 +682,7 @@ def render_filled(
         runtime.image = runtime.mymap.contourf(
             lons,
             lats,
-            self.data.field * self.data.fmult,
+            _mask_curvilinear_contour_seams(self.data) * self.data.fmult,
             self.data.levels,
             extend=scale.levels_extend,
             cmap=cmap,
@@ -792,10 +792,11 @@ def render_lines(
 
         runtime = plotvars.runtime
         dec = plotvars.decoration
+        field = _mask_curvilinear_contour_seams(self.data) * self.data.fmult
         cs = runtime.mymap.contour(
             self.data.x,
             self.data.y,
-            self.data.field * self.data.fmult,
+            field,
             self.data.levels,
             colors=colors,
             linewidths=linewidths,
@@ -825,7 +826,7 @@ def render_lines(
             cs0 = runtime.mymap.contour(
                 self.data.x,
                 self.data.y,
-                self.data.field * self.data.fmult,
+                field,
                 [-1e-32, 0],
                 colors=colors,
                 linewidths=zero_thick,
@@ -1697,6 +1698,70 @@ def _window_irregular_map_data(
     return field_irregular, lons_irregular, lats_irregular
 
 
+def _is_curvilinear_map_grid(data: ContourData) -> bool:
+    """Return True for map plots using 2D lon/lat coordinates.
+
+    Curvilinear grids (for example NEMO/ORCA ocean meshes) are prone to
+    severe contourf/contour artefacts. These are better rendered through
+    blockfill/pcolormesh.
+    """
+    if data.ptype != 1 or data.x is None or data.y is None:
+        return False
+    if np.ndim(data.field) != 2:
+        return False
+    return np.ndim(data.x) == 2 and np.ndim(data.y) == 2
+
+
+def _mask_curvilinear_contour_seams(data: ContourData) -> np.ndarray:
+    """Exclude vertices adjoining longitude jumps from structured contours."""
+    if not _is_curvilinear_map_grid(data):
+        return data.field
+
+    mask = np.ma.getmaskarray(data.field).copy()
+    for axis in (0, 1):
+        jumps = np.abs(np.diff(data.x, axis=axis)) > 180.0
+        before = [slice(None), slice(None)]
+        after = before.copy()
+        before[axis] = slice(None, -1)
+        after[axis] = slice(1, None)
+        # Mask both ends so corner masking cannot retain a seam-crossing
+        # triangle and interpolate a contour across the whole map.
+        mask[tuple(before)] |= jumps
+        mask[tuple(after)] |= jumps
+
+    return np.ma.array(data.field, mask=mask)
+
+
+def _resolve_curvilinear_render_defaults(
+    *,
+    data: ContourData,
+    kwargs: dict[str, Any],
+    fill: bool,
+    lines: bool,
+    blockfill: bool,
+    blockfill_fast: bool | None,
+) -> tuple[bool, bool, bool, bool | None]:
+    """Adjust renderer defaults for curvilinear lon/lat map grids.
+
+    Returns the possibly updated (fill, lines, blockfill, blockfill_fast)
+    tuple.
+    """
+    if not _is_curvilinear_map_grid(data):
+        return fill, lines, blockfill, blockfill_fast
+
+    # Use robust defaults for 2D map grids unless the caller explicitly
+    # requested alternative behavior.
+    if "blockfill" not in kwargs:
+        blockfill = True
+        fill = False
+    if blockfill and blockfill_fast is None:
+        blockfill_fast = True
+    if "lines" not in kwargs:
+        lines = False
+
+    return fill, lines, blockfill, blockfill_fast
+
+
 def _render_with_new_xy(
     f: Any, x: Any, y: Any, kwargs: dict[str, Any]
 ) -> bool:
@@ -1776,6 +1841,7 @@ def _render_with_new_xy(
     fill = kwargs.get("fill", global_fill)
     lines = kwargs.get("lines", global_lines)
     blockfill = kwargs.get("blockfill", global_blockfill)
+    blockfill_fast = kwargs.get("blockfill_fast", None)
     line_labels = kwargs.get("line_labels", True)
     zero_thick = kwargs.get("zero_thick", False)
     colors = kwargs.get("colors", "k")
@@ -1787,6 +1853,17 @@ def _render_with_new_xy(
     if blockfill:
         fill = False
 
+    fill, lines, blockfill, blockfill_fast = (
+        _resolve_curvilinear_render_defaults(
+            data=data,
+            kwargs=kwargs,
+            fill=fill,
+            lines=lines,
+            blockfill=blockfill,
+            blockfill_fast=blockfill_fast,
+        )
+    )
+
     colorbar = kwargs.get("colorbar", True)
     if not fill and not blockfill:
         colorbar = False
@@ -2171,7 +2248,7 @@ def _render_with_new_xy(
         )
     if blockfill:
         renderer.render_blockfill(
-            fast=kwargs.get("blockfill_fast", None), alpha=alpha, zorder=zorder
+            fast=blockfill_fast, alpha=alpha, zorder=zorder
         )
     if lines:
         renderer.render_lines(
@@ -2350,7 +2427,11 @@ def con(f=None, x=None, y=None, **kwargs):
     |                       6 = rotated pole
     | negative_linestyle='solid' - set to one of 'solid', 'dashed'
     | zero_thick=False - add a thick zero contour line. Set to 3 for example.
-    | blockfill=False - set to True for a blockfill plot
+    | blockfill=False - set to True for a blockfill plot. For 2D lon/lat
+    |                  grids, defaults to fast blockfill without lines.
+    |                  Explicit False selects contours; vertices adjoining
+    |                  longitude wrap jumps are excluded to avoid stripes,
+    |                  leaving a narrow gap at the seam.
     | colorbar_title=colbar_title - title for the colour bar
     | colorbar=True - add a colour bar if a filled contour plot
     | colorbar_label_skip=None - skip colour bar labels. Set to 2 to skip
diff --git a/docs/source/examples/example_1.rst b/docs/source/examples/example_1.rst
index d0600d87..146a6e10 100644
--- a/docs/source/examples/example_1.rst
+++ b/docs/source/examples/example_1.rst
@@ -14,4 +14,4 @@ Example 1: Basic contour plot in default projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_1.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_1.png
diff --git a/docs/source/examples/example_10.rst b/docs/source/examples/example_10.rst
index 63d61fe5..87586a29 100644
--- a/docs/source/examples/example_10.rst
+++ b/docs/source/examples/example_10.rst
@@ -14,4 +14,4 @@ Example 10: Latitude-time Hovmöller plot
    cfp.con(f.subspace(longitude=0), lines=0)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_10.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_10.png
diff --git a/docs/source/examples/example_11.rst b/docs/source/examples/example_11.rst
index b6b387cb..a93c4f75 100644
--- a/docs/source/examples/example_11.rst
+++ b/docs/source/examples/example_11.rst
@@ -18,4 +18,4 @@ Example 11: Latitude-time subset view Hovmöller plot
    cfp.con(f.subspace(longitude=0), lines=0)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_11.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_11.png
diff --git a/docs/source/examples/example_12.rst b/docs/source/examples/example_12.rst
index 3d8997c9..ec1b2049 100644
--- a/docs/source/examples/example_12.rst
+++ b/docs/source/examples/example_12.rst
@@ -14,4 +14,4 @@ Example 12: Longitude-time Hovmöller plot
    cfp.con(f.subspace(latitude=0), lines=0)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_12.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_12.png
diff --git a/docs/source/examples/example_13.rst b/docs/source/examples/example_13.rst
index d2409c9d..55371e54 100644
--- a/docs/source/examples/example_13.rst
+++ b/docs/source/examples/example_13.rst
@@ -18,4 +18,4 @@ Example 13: Basic vector plot
 
    cfp.vect(u=u, v=v, key_length=10, scale=100, stride=5)
 
-.. figure:: /../../tests/reference-example-images/ref_fig_13.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_13.png
diff --git a/docs/source/examples/example_14.rst b/docs/source/examples/example_14.rst
index 8025a452..865dca79 100644
--- a/docs/source/examples/example_14.rst
+++ b/docs/source/examples/example_14.rst
@@ -26,4 +26,4 @@ Example 14: Vector plot overlaid on a contour map
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_14.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_14.png
diff --git a/docs/source/examples/example_15.rst b/docs/source/examples/example_15.rst
index e00cbf84..6da06aac 100644
--- a/docs/source/examples/example_15.rst
+++ b/docs/source/examples/example_15.rst
@@ -37,4 +37,4 @@ Example 15: Polar projection vector plot
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_15.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_15.png
diff --git a/docs/source/examples/example_16a.rst b/docs/source/examples/example_16a.rst
index 55661137..5b7a5e58 100644
--- a/docs/source/examples/example_16a.rst
+++ b/docs/source/examples/example_16a.rst
@@ -29,4 +29,4 @@ Example 16a: Zonal vector plot
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_16a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_16a.png
diff --git a/docs/source/examples/example_16b.rst b/docs/source/examples/example_16b.rst
index ce5d92ef..2323e042 100644
--- a/docs/source/examples/example_16b.rst
+++ b/docs/source/examples/example_16b.rst
@@ -21,4 +21,4 @@ Example 16b: Basic stream plot
    cfp.stream(u=u, v=v, density=2)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_16b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_16b.png
diff --git a/docs/source/examples/example_16c.rst b/docs/source/examples/example_16c.rst
index ef1dcf11..f76c43ee 100644
--- a/docs/source/examples/example_16c.rst
+++ b/docs/source/examples/example_16c.rst
@@ -34,4 +34,4 @@ Example 16c: Stream plot in a colour scale
    )
    cfp.gclose()
 
-.. figure:: /../../tests/reference-example-images/ref_fig_16c.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_16c.png
diff --git a/docs/source/examples/example_17.rst b/docs/source/examples/example_17.rst
index b3513df2..b10811e4 100644
--- a/docs/source/examples/example_17.rst
+++ b/docs/source/examples/example_17.rst
@@ -22,4 +22,4 @@ Example 17: Basic stipple plot
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_17.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_17.png
diff --git a/docs/source/examples/example_18.rst b/docs/source/examples/example_18.rst
index 274bd933..66802eac 100644
--- a/docs/source/examples/example_18.rst
+++ b/docs/source/examples/example_18.rst
@@ -19,4 +19,4 @@ Example 18: Polar stipple plot
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_18.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_18.png
diff --git a/docs/source/examples/example_19a.rst b/docs/source/examples/example_19a.rst
index 1cabefb8..6f46b760 100644
--- a/docs/source/examples/example_19a.rst
+++ b/docs/source/examples/example_19a.rst
@@ -29,4 +29,4 @@ Example 19a: Multiple plots
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_19a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_19a.png
diff --git a/docs/source/examples/example_19b.rst b/docs/source/examples/example_19b.rst
index 730a2a73..55d5e3df 100644
--- a/docs/source/examples/example_19b.rst
+++ b/docs/source/examples/example_19b.rst
@@ -25,4 +25,4 @@ Example 19b: Multiple plots with user-specified plot positions
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_19b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_19b.png
diff --git a/docs/source/examples/example_19c.rst b/docs/source/examples/example_19c.rst
index 650e1424..e7599af8 100644
--- a/docs/source/examples/example_19c.rst
+++ b/docs/source/examples/example_19c.rst
@@ -39,4 +39,4 @@ Example 19c: Accomodating more than one colour bar
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_19c.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_19c.png
diff --git a/docs/source/examples/example_2.rst b/docs/source/examples/example_2.rst
index 3f166095..a1e1aa31 100644
--- a/docs/source/examples/example_2.rst
+++ b/docs/source/examples/example_2.rst
@@ -13,4 +13,4 @@ Example 2: Basic blockfill plot in default projection
    cfp.con(f.subspace(time=15), blockfill=True, lines=False)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_2.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_2.png
diff --git a/docs/source/examples/example_20.rst b/docs/source/examples/example_20.rst
index 5edf0808..8f937246 100644
--- a/docs/source/examples/example_20.rst
+++ b/docs/source/examples/example_20.rst
@@ -15,4 +15,4 @@ Example 20: Case where user-defined axis labels are required
    cfp.con(f.subspace[9])
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_20.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_20.png
diff --git a/docs/source/examples/example_21a.rst b/docs/source/examples/example_21a.rst
index 6f4a6dc6..ed26cbcb 100644
--- a/docs/source/examples/example_21a.rst
+++ b/docs/source/examples/example_21a.rst
@@ -21,4 +21,4 @@ Example 21a: User-defined axes
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_21a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_21a.png
diff --git a/docs/source/examples/example_21b.rst b/docs/source/examples/example_21b.rst
index 25f348d1..615eb0ea 100644
--- a/docs/source/examples/example_21b.rst
+++ b/docs/source/examples/example_21b.rst
@@ -14,4 +14,4 @@ Example 21b: Plot of rotated pole data
 
    cfp.con(f)
 
-.. figure:: /../../tests/reference-example-images/ref_fig_21b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_21b.png
diff --git a/docs/source/examples/example_22.rst b/docs/source/examples/example_22.rst
index 23d9592a..3ee27771 100644
--- a/docs/source/examples/example_22.rst
+++ b/docs/source/examples/example_22.rst
@@ -16,4 +16,4 @@ Example 22: Plot of rotated pole data on its native grid
    cfp.con(f)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_22.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_22.png
diff --git a/docs/source/examples/example_23a.rst b/docs/source/examples/example_23a.rst
index d843943d..ead289d6 100644
--- a/docs/source/examples/example_23a.rst
+++ b/docs/source/examples/example_23a.rst
@@ -30,4 +30,4 @@ Example 23a: Rotated pole plot from data which is not CF Compliant
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_23a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_23a.png
diff --git a/docs/source/examples/example_23b.rst b/docs/source/examples/example_23b.rst
index 764dcc0c..166ac7b8 100644
--- a/docs/source/examples/example_23b.rst
+++ b/docs/source/examples/example_23b.rst
@@ -23,4 +23,4 @@ Example 23b: Overlaying vectors over a rotated pole data plot
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_23b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_23b.png
diff --git a/docs/source/examples/example_24a.rst b/docs/source/examples/example_24a.rst
index fbd4f0ee..bf2b3600 100644
--- a/docs/source/examples/example_24a.rst
+++ b/docs/source/examples/example_24a.rst
@@ -28,4 +28,4 @@ Example 24a: UGRID blockfill plot with LFRic cubed sphere mesh output
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_24a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_24a.png
diff --git a/docs/source/examples/example_24b.rst b/docs/source/examples/example_24b.rst
index fe4d574e..f5d24396 100644
--- a/docs/source/examples/example_24b.rst
+++ b/docs/source/examples/example_24b.rst
@@ -30,4 +30,4 @@ Example 24b: UGRID plot in the polar stereographic projection view
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_24b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_24b.png
diff --git a/docs/source/examples/example_24c.rst b/docs/source/examples/example_24c.rst
index 8dd0bb6e..c023ab69 100644
--- a/docs/source/examples/example_24c.rst
+++ b/docs/source/examples/example_24c.rst
@@ -17,4 +17,4 @@ Example 24c: UGRID contour plot with LFRic cubed sphere mesh output
    cfp.con(g, lines=False)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_24c.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_24c.png
diff --git a/docs/source/examples/example_25.rst b/docs/source/examples/example_25.rst
index 98642692..90cf834f 100644
--- a/docs/source/examples/example_25.rst
+++ b/docs/source/examples/example_25.rst
@@ -26,4 +26,4 @@ Example 25: UGRID contour plot with ORCA 2 output
    cfp.con(f=temp.array, x=lons.array, y=lats.array, ptype=1)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_25.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_25.png
diff --git a/docs/source/examples/example_26a.rst b/docs/source/examples/example_26a.rst
index 102f4e95..46a938c9 100644
--- a/docs/source/examples/example_26a.rst
+++ b/docs/source/examples/example_26a.rst
@@ -29,4 +29,4 @@ Example 26a: Contour plot based on discrete feature values
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_26a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_26a.png
diff --git a/docs/source/examples/example_26b.rst b/docs/source/examples/example_26b.rst
index 1ca8aecc..23929da7 100644
--- a/docs/source/examples/example_26b.rst
+++ b/docs/source/examples/example_26b.rst
@@ -43,4 +43,4 @@ Example 26b: Contour plot from discrete feature values with labelling
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_26b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_26b.png
diff --git a/docs/source/examples/example_27.rst b/docs/source/examples/example_27.rst
index 5f1f8bef..970d322b 100644
--- a/docs/source/examples/example_27.rst
+++ b/docs/source/examples/example_27.rst
@@ -22,4 +22,4 @@ Example 27: Basic line plot
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_27.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_27.png
diff --git a/docs/source/examples/example_28.rst b/docs/source/examples/example_28.rst
index ec7695f6..a700cfa9 100644
--- a/docs/source/examples/example_28.rst
+++ b/docs/source/examples/example_28.rst
@@ -58,4 +58,4 @@ Example 28: Line plot with a legend
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_28.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_28.png
diff --git a/docs/source/examples/example_29.rst b/docs/source/examples/example_29.rst
index 54164029..7bfbd599 100644
--- a/docs/source/examples/example_29.rst
+++ b/docs/source/examples/example_29.rst
@@ -22,4 +22,4 @@ Example 29: Time series line plot
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_29.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_29.png
diff --git a/docs/source/examples/example_3.rst b/docs/source/examples/example_3.rst
index ead1d5f2..4b232ebf 100644
--- a/docs/source/examples/example_3.rst
+++ b/docs/source/examples/example_3.rst
@@ -17,4 +17,4 @@ Example 3: Contour plot with altered map limits and levels
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_3.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_3.png
diff --git a/docs/source/examples/example_30.rst b/docs/source/examples/example_30.rst
index 4dc0fd95..caa76705 100644
--- a/docs/source/examples/example_30.rst
+++ b/docs/source/examples/example_30.rst
@@ -31,4 +31,4 @@ Example 30: Line plot with two x axes
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_30.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_30.png
diff --git a/docs/source/examples/example_31.rst b/docs/source/examples/example_31.rst
index d55f6c35..96c8ef3b 100644
--- a/docs/source/examples/example_31.rst
+++ b/docs/source/examples/example_31.rst
@@ -15,4 +15,4 @@ Example 31: UKCP projection
 
    cfp.con(f, lines=False)
 
-.. figure:: /../../tests/reference-example-images/ref_fig_31.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_31.png
diff --git a/docs/source/examples/example_32.rst b/docs/source/examples/example_32.rst
index 26ad8a20..9dcf2066 100644
--- a/docs/source/examples/example_32.rst
+++ b/docs/source/examples/example_32.rst
@@ -23,4 +23,4 @@ Example 32: UKCP projection with blockfill
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_32.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_32.png
diff --git a/docs/source/examples/example_33.rst b/docs/source/examples/example_33.rst
index 36d932a0..4cd84732 100644
--- a/docs/source/examples/example_33.rst
+++ b/docs/source/examples/example_33.rst
@@ -20,4 +20,4 @@ Example 33: OSGB and EuroPP projections
    cfp.gclose()
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_33.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_33.png
diff --git a/docs/source/examples/example_34.rst b/docs/source/examples/example_34.rst
index 577690fe..808f7166 100644
--- a/docs/source/examples/example_34.rst
+++ b/docs/source/examples/example_34.rst
@@ -15,4 +15,4 @@ Example 34: Cropping the Lambert Conformal Conic (LCC) projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_34.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_34.png
diff --git a/docs/source/examples/example_35.rst b/docs/source/examples/example_35.rst
index dd64d5af..33cc427d 100644
--- a/docs/source/examples/example_35.rst
+++ b/docs/source/examples/example_35.rst
@@ -14,4 +14,4 @@ Example 35: Mollweide projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_35.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_35.png
diff --git a/docs/source/examples/example_36.rst b/docs/source/examples/example_36.rst
index 1f2176e9..7ff85186 100644
--- a/docs/source/examples/example_36.rst
+++ b/docs/source/examples/example_36.rst
@@ -14,4 +14,4 @@ Example 36: Mercator projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_36.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_36.png
diff --git a/docs/source/examples/example_37.rst b/docs/source/examples/example_37.rst
index 156c31bd..55fa5f5e 100644
--- a/docs/source/examples/example_37.rst
+++ b/docs/source/examples/example_37.rst
@@ -14,4 +14,4 @@ Example 37: Orthographic projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_37.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_37.png
diff --git a/docs/source/examples/example_38.rst b/docs/source/examples/example_38.rst
index 36c6353e..100c59d0 100644
--- a/docs/source/examples/example_38.rst
+++ b/docs/source/examples/example_38.rst
@@ -14,4 +14,4 @@ Example 38: Robinson projection
    cfp.con(f.subspace(time=15))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_38.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_38.png
diff --git a/docs/source/examples/example_39.rst b/docs/source/examples/example_39.rst
index ff18b206..11398441 100644
--- a/docs/source/examples/example_39.rst
+++ b/docs/source/examples/example_39.rst
@@ -12,4 +12,4 @@ Example 39: Basic track plotting
    cfp.traj(f)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_39.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_39.png
diff --git a/docs/source/examples/example_39b.rst b/docs/source/examples/example_39b.rst
index 8c2d3224..d5070c3c 100644
--- a/docs/source/examples/example_39b.rst
+++ b/docs/source/examples/example_39b.rst
@@ -15,4 +15,4 @@ Example 39b: plotting a single (1D) DSG trajectory
    cfp.traj(f)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_39b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_39b.png
diff --git a/docs/source/examples/example_4.rst b/docs/source/examples/example_4.rst
index e3e96229..4a752df9 100644
--- a/docs/source/examples/example_4.rst
+++ b/docs/source/examples/example_4.rst
@@ -16,4 +16,4 @@ Example 4: North Pole polar stereographic projection contour plot
    cfp.con(f.subspace(pressure=500))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_4.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_4.png
diff --git a/docs/source/examples/example_40.rst b/docs/source/examples/example_40.rst
index 8be4aefd..614c868c 100644
--- a/docs/source/examples/example_40.rst
+++ b/docs/source/examples/example_40.rst
@@ -15,4 +15,4 @@ Example 40: Tracks in the polar stereographic projection
    cfp.traj(f)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_40.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_40.png
diff --git a/docs/source/examples/example_41.rst b/docs/source/examples/example_41.rst
index 97e70e64..ef7459b6 100644
--- a/docs/source/examples/example_41.rst
+++ b/docs/source/examples/example_41.rst
@@ -14,4 +14,4 @@ Example 41: Feature propagation over Europe
    cfp.traj(f, vector=True, markersize=0.0, fc="b", ec="b")
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_41.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_41.png
diff --git a/docs/source/examples/example_42a.rst b/docs/source/examples/example_42a.rst
index d6c3e9f3..a69affc6 100644
--- a/docs/source/examples/example_42a.rst
+++ b/docs/source/examples/example_42a.rst
@@ -26,4 +26,4 @@ Example 42a: Tracks with labelled data points
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_42a.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_42a.png
diff --git a/docs/source/examples/example_42b.rst b/docs/source/examples/example_42b.rst
index b11ae6e4..818a7ed8 100644
--- a/docs/source/examples/example_42b.rst
+++ b/docs/source/examples/example_42b.rst
@@ -25,4 +25,4 @@ Example 42b: Tracks displayed in a colour scale
    )
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_42b.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_42b.png
diff --git a/docs/source/examples/example_5.rst b/docs/source/examples/example_5.rst
index 8675a43c..4256884e 100644
--- a/docs/source/examples/example_5.rst
+++ b/docs/source/examples/example_5.rst
@@ -17,4 +17,4 @@ Example 5: South Pole polar projection contour plot with bounding latitude
    cfp.con(f.subspace(pressure=500))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_5.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_5.png
diff --git a/docs/source/examples/example_6.rst b/docs/source/examples/example_6.rst
index 7e7402cc..492d8c5b 100644
--- a/docs/source/examples/example_6.rst
+++ b/docs/source/examples/example_6.rst
@@ -14,4 +14,4 @@ Example 6: Latitude-pressure plot at set longitude
    cfp.con(f.subspace(longitude=0))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_6.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_6.png
diff --git a/docs/source/examples/example_7.rst b/docs/source/examples/example_7.rst
index 58dbe486..a8f9e113 100644
--- a/docs/source/examples/example_7.rst
+++ b/docs/source/examples/example_7.rst
@@ -14,4 +14,4 @@ Example 7: Latitude-pressure plot over zonal mean
    cfp.con(f.collapse("mean", "longitude"))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_7.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_7.png
diff --git a/docs/source/examples/example_8.rst b/docs/source/examples/example_8.rst
index aaa80fee..cb63ed25 100644
--- a/docs/source/examples/example_8.rst
+++ b/docs/source/examples/example_8.rst
@@ -14,4 +14,4 @@ Example 8: Latitude against log of pressure over longitude zonal mean
    cfp.con(f.collapse("mean", "longitude"), ylog=1)
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_8.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_8.png
diff --git a/docs/source/examples/example_9.rst b/docs/source/examples/example_9.rst
index d450a519..f1e3e51f 100644
--- a/docs/source/examples/example_9.rst
+++ b/docs/source/examples/example_9.rst
@@ -14,4 +14,4 @@ Example 9: Longitude-pressure plot over latitude mean
    cfp.con(f.collapse("mean", "latitude"))
 
 
-.. figure:: /../../tests/reference-example-images/ref_fig_9.png
+.. figure:: /../../tests/v350-controlled-env-ref-images/ref_fig_9.png
diff --git a/docs/source/gallery_of_examples.rst b/docs/source/gallery_of_examples.rst
index 67b285ae..467c1bd4 100644
--- a/docs/source/gallery_of_examples.rst
+++ b/docs/source/gallery_of_examples.rst
@@ -40,77 +40,77 @@ by clicking through to the category pages.
    :gutter: 1
 
    .. grid-item-card:: Contour plots
-      :img-top: /../../tests/reference-example-images/ref_fig_1.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_1.png
       :link: cylindrical.html#example-1-basic-contour-plot-in-default-projection
       :text-align: center
 
    .. grid-item-card:: Blockfill plots
-      :img-top: /../../tests/reference-example-images/ref_fig_2.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_2.png
       :link: cylindrical.html#example-2-basic-blockfill-plot-in-default-projection
       :text-align: center
 
    .. grid-item-card:: Vector plots
-      :img-top: /../../tests/reference-example-images/ref_fig_13.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_13.png
       :link: vectors.html#example-13-basic-vector-plot
       :text-align: center
 
    .. grid-item-card:: Stream plots
-      :img-top: /../../tests/reference-example-images/ref_fig_16c.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_16c.png
       :link: vectors.html#example-16c-stream-plot-in-a-colour-scale
       :text-align: center
 
    .. grid-item-card:: Stipple plots
-      :img-top: /../../tests/reference-example-images/ref_fig_17.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_17.png
       :link: stipple_plots.html#example-17-basic-stipple-plot
       :text-align: center
 
    .. grid-item-card:: Trajectories
-      :img-top: /../../tests/reference-example-images/ref_fig_42b.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_42b.png
       :link: trajectories.html#example-42b-tracks-displayed-in-a-colour-scale
       :text-align: center
 
    .. grid-item-card:: Hovmöller plots
-      :img-top: /../../tests/reference-example-images/ref_fig_11.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_11.png
       :link: hovmuller.html#example-11-latitude-time-subset-view-hovmoller-plot
       :text-align: center
 
    .. grid-item-card:: Vertical (pressure or height) plots
-      :img-top: /../../tests/reference-example-images/ref_fig_8.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_8.png
       :link: pressure.html#example-8-latitude-against-log-of-pressure-over-longitude-zonal-mean
       :text-align: center
 
    .. grid-item-card:: Lineplots
-      :img-top: /../../tests/reference-example-images/ref_fig_27.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_27.png
       :link: graphs.html#example-27-basic-line-plot
       :text-align: center
 
    .. grid-item-card:: Support for multiple projections
-      :img-top: /../../tests/reference-example-images/ref_fig_34.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_34.png
       :link: projections.html#example-34-cropping-the-lambert-conformal-conic-lcc-projection
       :text-align: center
 
    .. grid-item-card:: Polar projection views
-      :img-top: /../../tests/reference-example-images/ref_fig_40.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_40.png
       :link: trajectories.html#example-40-tracks-in-the-polar-stereographic-projection
       :text-align: center
 
    .. grid-item-card:: Unstructured grids (UGRID) support
-      :img-top: /../../tests/reference-example-images/ref_fig_24a.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_24a.png
       :link: unstructured.html#example-24a-ugrid-blockfill-plot-with-lfric-cubed-sphere-mesh-output
       :text-align: center
 
    .. grid-item-card:: Rotated pole support
-      :img-top: /../../tests/reference-example-images/ref_fig_22.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_22.png
       :link: rotated_pole.html#example-22-plot-of-rotated-pole-data-on-its-native-grid
       :text-align: center
 
    .. grid-item-card:: Multiple plots on one figure
-      :img-top: /../../tests/reference-example-images/ref_fig_19b.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_19b.png
       :link: multiple_plots.html#example-19b-multiple-plots-with-user-specified-plot-positions
       :text-align: center
 
    .. grid-item-card:: Flexible customisation e.g. user-defined axes
-      :img-top: /../../tests/reference-example-images/ref_fig_21a.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_21a.png
       :link: user_defined.html#example-21a-user-defined-axes
       :text-align: center
 
diff --git a/tests/data/nemo_eg260916.nc b/tests/data/nemo_eg260916.nc
new file mode 100644
index 00000000..87cee8b4
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diff --git a/tests/data/qrparm.c48.orog.nc b/tests/data/qrparm.c48.orog.nc
new file mode 100644
index 00000000..f9e80283
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diff --git a/tests/integration/test_contour_plot_examples.py b/tests/integration/test_contour_plot_examples.py
index a44c78f1..426eebd0 100644
--- a/tests/integration/test_contour_plot_examples.py
+++ b/tests/integration/test_contour_plot_examples.py
@@ -684,6 +684,22 @@ def test_example_37b_orthographic_full_globe():
     _assert_reference_match("37b")
 
 
+@pytest.mark.integration
+def test_example_nemo_1():
+    """Test plotting of data on a NEMO grid."""
+    data_file = Path(__file__).parent.parent / "data" / "nemo_eg260916.nc"
+    if not data_file.exists():
+        pytest.skip(f"Missing test data: {data_file}")
+
+    fl = cf.read(str(data_file))
+    f = fl[0].squeeze()
+
+    _configure_example_output("nemo1")
+    cfp.con(f)
+
+    _assert_reference_match("nemo1")
+
+
 @pytest.mark.integration
 def test_example_numpy_arrays():
     """Test contour with raw numpy arrays."""
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diff --git a/tests/unit/test_contour_boundary.py b/tests/unit/test_contour_boundary.py
index 55172961..c82c1863 100644
--- a/tests/unit/test_contour_boundary.py
+++ b/tests/unit/test_contour_boundary.py
@@ -1,5 +1,9 @@
 import inspect
+from pathlib import Path
+from types import SimpleNamespace
 
+import cf
+import contourpy
 import numpy as np
 import pytest
 
@@ -88,3 +92,188 @@ def test_blockfill_signature_does_not_advertise_lonlat():
     params = inspect.signature(blockfill._bfill).parameters
 
     assert "lonlat" not in params
+
+
+def test_curvilinear_defaults_enable_fast_blockfill_and_disable_lines():
+    data = contour.ContourData(
+        field=np.ones((3, 4)),
+        x=np.ones((3, 4)),
+        y=np.ones((3, 4)),
+        ptype=1,
+    )
+
+    fill, lines, blockfill, blockfill_fast = contour._resolve_curvilinear_render_defaults(
+        data=data,
+        kwargs={},
+        fill=True,
+        lines=True,
+        blockfill=False,
+        blockfill_fast=None,
+    )
+
+    assert fill is False
+    assert lines is False
+    assert blockfill is True
+    assert blockfill_fast is True
+
+
+def test_curvilinear_defaults_respect_explicit_user_choices():
+    data = contour.ContourData(
+        field=np.ones((3, 4)),
+        x=np.ones((3, 4)),
+        y=np.ones((3, 4)),
+        ptype=1,
+    )
+
+    fill, lines, blockfill, blockfill_fast = contour._resolve_curvilinear_render_defaults(
+        data=data,
+        kwargs={"blockfill": False, "lines": True},
+        fill=True,
+        lines=True,
+        blockfill=False,
+        blockfill_fast=None,
+    )
+
+    assert fill is True
+    assert lines is True
+    assert blockfill is False
+    assert blockfill_fast is None
+
+
+@pytest.mark.parametrize("axis", [0, 1])
+def test_curvilinear_contours_mask_both_sides_of_seam(axis):
+    x = np.tile([178.0, 179.0, -179.0, -178.0], (4, 1))
+    if axis == 0:
+        x = x.T
+    field = np.ma.array(np.ones((4, 4)), mask=False)
+    field.mask[0, 0] = True
+    data = contour.ContourData(
+        field=field, x=x, y=np.ones((4, 4)), ptype=1
+    )
+
+    result = contour._mask_curvilinear_contour_seams(data)
+
+    expected = np.zeros((4, 4), dtype=bool)
+    if axis == 0:
+        expected[1:3, :] = True
+    else:
+        expected[:, 1:3] = True
+    expected[0, 0] = True
+    np.testing.assert_array_equal(result.mask, expected)
+    np.testing.assert_array_equal(result.data, field.data)
+    assert np.ma.count_masked(field) == 1
+
+
+@pytest.mark.parametrize("curvilinear", [False, True])
+def test_contour_seam_mask_preserves_continuous_grids(curvilinear):
+    field = np.arange(12.0).reshape(3, 4)
+    x, y = np.arange(4.0), np.arange(3.0)
+    if curvilinear:
+        x, y = np.meshgrid(x, y)
+    data = contour.ContourData(field=field, x=x, y=y, ptype=1)
+
+    result = contour._mask_curvilinear_contour_seams(data)
+
+    np.testing.assert_array_equal(result, field)
+    assert np.ma.count_masked(result) == 0
+
+
+def test_map_renderers_use_seam_mask_for_fill_lines_and_zero(monkeypatch):
+    calls = []
+
+    def record(x, y, field, levels, **kwargs):
+        calls.append(field)
+        return SimpleNamespace()
+
+    monkeypatch.setattr(
+        contour.plotvars.runtime,
+        "mymap",
+        SimpleNamespace(contourf=record, contour=record),
+    )
+    data = contour.ContourData(
+        field=np.ones((3, 4)),
+        x=np.tile([178.0, 179.0, -179.0, -178.0], (3, 1)),
+        y=np.tile(np.arange(3.0)[:, None], (1, 4)),
+        ptype=1,
+        levels=np.array([-1.0, 0.0, 1.0]),
+        fmult=2,
+    )
+    renderer = contour.MapContourRenderer(
+        layout=None,
+        data=data,
+        colour_scale=SimpleNamespace(get_cmap=lambda: None),
+    )
+
+    renderer.render_filled(alpha=1, zorder=1, transform_first=False)
+    renderer.render_lines(
+        colors="k",
+        linewidths=None,
+        linestyles=None,
+        line_labels=False,
+        zero_thick=3,
+    )
+
+    assert len(calls) == 3
+    for field in calls:
+        np.testing.assert_array_equal(field.mask[:, 1:3], True)
+        np.testing.assert_array_equal(field.compressed(), 2.0)
+    assert np.ma.count_masked(data.field) == 0
+
+
+def test_map_blockfill_does_not_apply_contour_seam_mask(monkeypatch):
+    calls = []
+    monkeypatch.setattr(
+        contour, "_bfill", lambda **kwargs: calls.append(kwargs["f"])
+    )
+    data = contour.ContourData(
+        field=np.ones((3, 4)),
+        x=np.tile([178.0, 179.0, -179.0, -178.0], (3, 1)),
+        y=np.ones((3, 4)),
+        ptype=1,
+        levels=np.array([0.0, 1.0]),
+    )
+    renderer = contour.MapContourRenderer(
+        layout=None, data=data, colour_scale=None
+    )
+
+    renderer.render_blockfill(fast=True, alpha=1, zorder=1)
+
+    assert len(calls) == 1
+    assert np.ma.count_masked(calls[0]) == 0
+    np.testing.assert_array_equal(calls[0], data.field)
+
+
+@pytest.mark.parametrize("filled", [False, True])
+def test_nemo_contours_do_not_interpolate_across_longitude_seam(filled):
+    path = Path(__file__).parents[1] / "data" / "nemo_eg260916.nc"
+    data = contour.ContourData.from_cf_field(cf.read(str(path))[0], None)
+    field = contour._mask_curvilinear_contour_seams(data)
+    levels = np.array([0.5, 1.5, 2.5, 3.5]) * 1e-4
+
+    def longest_edge(values):
+        generator = contourpy.contour_generator(
+            x=data.x, y=data.y, z=values, corner_mask=True
+        )
+        if filled:
+            paths = []
+            for lower, upper in zip(levels[:-1], levels[1:]):
+                vertices, offsets = generator.filled(lower, upper)
+                for points, boundaries in zip(vertices, offsets):
+                    paths.extend(
+                        points[start:end]
+                        for start, end in zip(
+                            boundaries[:-1], boundaries[1:]
+                        )
+                    )
+        else:
+            paths = [
+                path for level in levels for path in generator.lines(level)
+            ]
+        return max(
+            np.max(np.abs(np.diff(path[:, 0])))
+            for path in paths
+            if len(path) > 1
+        )
+
+    assert longest_edge(data.field) > 300.0
+    assert longest_edge(field) < 180.0
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