Fix bug with set_boundary() and clipping (Fixes SciTools#1620)

We weren't set up to be able to update the transform and have that
propagate downstream because other parts of matplotlib grab a reference
to the Axes.patch transform. Therefore, use a TransformWrapper so we can
update.

lib/cartopy/mpl/geoaxes.py

@@ -235,14 +235,25 @@ def __init__(self, axes, **kwargs):
         super().__init__(self._original_path, **kwargs)
         self._axes = axes
 
+        # We need to use a TransformWrapper as our transform so that we can
+        # update the transform without breaking others' references to this one.
+        self._trans_wrap = mtransforms.TransformWrapper(self.get_transform())
+
+    def set_transform(self, transform):
+        self._trans_wrap.set(transform)
+        super().set_transform(self._trans_wrap)
+
     def set_boundary(self, path, transform):
         self._original_path = path
         self.set_transform(transform)
         self.stale = True
 
     def _adjust_location(self):
         if self.stale:
-            self._path = self._original_path.clip_to_bbox(self.axes.viewLim)
+            self.set_path(self._original_path.clip_to_bbox(self.axes.viewLim))
+            # Some places in matplotlib's tranform stack cache the actual
+            # path so we trigger an update by invalidating the transform.
+            self._trans_wrap.invalidate()
 
     @matplotlib.artist.allow_rasterization
     def draw(self, renderer, *args, **kwargs):

lib/cartopy/tests/mpl/test_axes.py

@@ -132,3 +132,21 @@ def test_geoaxes_no_subslice():
         lats = np.linspace(35, 37, num_points)
         lons = np.linspace(-117, -115, num_points)
         ax.plot(lons, lats, transform=ccrs.PlateCarree())
+
+
+@ImageTesting(['geoaxes_set_boundary_clipping'])
+def test_geoaxes_set_boundary_clipping():
+    """Test that setting the boundary works properly for clipping #1620."""
+    lon, lat = np.meshgrid(np.linspace(-180., 180., 361),
+                           np.linspace(-90., -60., 31))
+    fig = plt.figure()
+    ax1 = fig.add_subplot(1, 1, 1, projection=ccrs.SouthPolarStereo())
+
+    # Limit the map to -60 degrees latitude and below.
+    ax1.set_extent([-180, 180, -90, -60], ccrs.PlateCarree())
+    ax1.gridlines()
+
+    ax1.contourf(lon, lat, lat, transform=ccrs.PlateCarree())
+
+    ax1.set_boundary(mpath.Path.circle(center=(0.5, 0.5), radius=0.5),
+                     transform=ax1.transAxes)