Avoid checking limits when updating both min and max for contours

closes matplotlib#26531

Author: ksunden

lib/matplotlib/contour.py

@@ -887,11 +887,12 @@ def __init__(self, ax, *args,
         self.set_cmap(cmap)
         if norm is not None:
             self.set_norm(norm)
-        if vmin is not None:
-            self.norm.vmin = vmin
-        if vmax is not None:
-            self.norm.vmax = vmax
-        self._process_colors()
+        with self.norm.callbacks.blocked(signal="changed"):
+            if vmin is not None:
+                self.norm.vmin = vmin
+            if vmax is not None:
+                self.norm.vmax = vmax
+            self._process_colors()
 
         if self._paths is None:
             self._paths = self._make_paths_from_contour_generator()