diff --git a/src/calibre/gui2/dialogs/quickview.py b/src/calibre/gui2/dialogs/quickview.py
index 97feb46dc5..b9386ba2a7 100644
--- a/src/calibre/gui2/dialogs/quickview.py
+++ b/src/calibre/gui2/dialogs/quickview.py
@@ -490,7 +490,9 @@ class Quickview(QDialog, Ui_Quickview):
             self.indicate_no_items()
 
     def is_category(self, key):
-        return key is not None and self.fm[key]['is_category']
+        return key is not None and (self.fm[key]['is_category'] or
+                                    (self.fm[key]['datatype'] == 'composite' and
+                                     self.fm[key]['display'].get('make_category', False)))
 
     def _refresh(self, book_id, key):
         '''
diff --git a/src/calibre/utils/formatter_functions.py b/src/calibre/utils/formatter_functions.py
index 720a563f40..e0513c2922 100644
--- a/src/calibre/utils/formatter_functions.py
+++ b/src/calibre/utils/formatter_functions.py
@@ -1306,16 +1306,9 @@ class BuiltinListUnion(BuiltinFormatterFunction):
     aliases = ['merge_lists']
 
     def evaluate(self, formatter, kwargs, mi, locals, list1, list2, separator):
-        res = [l.strip() for l in list1.split(separator) if l.strip()]
-        l2 = [l.strip() for l in list2.split(separator) if l.strip()]
-        lcl1 = {icu_lower(l) for l in res}
-
-        for i in l2:
-            if icu_lower(i) not in lcl1 and i not in res:
-                res.append(i)
-        if separator == ',':
-            return ', '.join(res)
-        return separator.join(res)
+        res = {icu_lower(l.strip()): l.strip() for l in list2.split(separator) if l.strip()}
+        res.update({icu_lower(l.strip()): l.strip() for l in list1.split(separator) if l.strip()})
+        return separator.join(res.values())
 
 
 class BuiltinListDifference(BuiltinFormatterFunction):