Add visual context on tree view

issue8562
review261951002

statement.py

@@ -409,6 +409,12 @@ class Statement(Workflow, ModelSQL, ModelView):
         for key, lines in groupby(lines, key=self._group_key):
             yield Line(**dict(key + (('lines', list(lines)),)))
 
+    @classmethod
+    def view_attributes(cls):
+        return [
+            ('/tree', 'visual', If(Eval('state') == 'cancel', 'muted', '')),
+            ]
+
     @classmethod
     def delete(cls, statements):
         # Cancel before delete