Fix the modindex checkboxes if none is selected.

sphinx/templates/modindex.html

@@ -19,6 +19,7 @@
             {%- if pl in showpf %} checked="checked"{% endif %}>
      <label for="pl-{{ pl }}">{{ pl }}</label>
      {% endfor %}
+     <input type="hidden" name="newpf" value="true">
      <input type="submit" value="Apply">
    </form>
 {% endif %}

sphinx/web/application.py

@@ -348,7 +348,7 @@ class DocumentationApplication(object):
                 } for x in sorted(most_frequent)]
 
         showpf = None
-        newpf = req.args.get('pf')
+        newpf = req.args.get('newpf')
         sesspf = req.session.get('pf')
         if newpf or sesspf:
             yield NoCache