keep the old buffer so we can return something when get_diff_image is called while self._png_is_old

cknd · cknd · commit edb58683d1ca · 2017-05-23T15:05:47.000+02:00
diff --git a/lib/matplotlib/backends/backend_webagg_core.py b/lib/matplotlib/backends/backend_webagg_core.py
@@ -236,8 +236,8 @@ def get_diff_image(self):
                 self._last_renderer, renderer)
             self._force_full = False
             self._png_is_old = False
-
-        return buff
+            self.buff = buff
+        return self.buff
 
     def get_renderer(self, cleared=None):
         # Mirrors super.get_renderer, but caches the old one
