--- a/hgext3rd/topic/__init__.py	Thu Oct 19 18:17:47 2017 +0200
+++ b/hgext3rd/topic/__init__.py	Thu Oct 19 18:18:43 2017 +0200
@@ -67,6 +67,7 @@
 
 from __future__ import absolute_import
 
+import functools
 import re
 import time
 import weakref
@@ -144,6 +145,38 @@
 minimumhgversion = '4.0'
 buglink = 'https://bz.mercurial-scm.org/'
 
+if util.safehasattr(registrar, 'configitem'):
+    configtable = {}
+    configitem = registrar.configitem(configtable)
+
+    configitem('experimental', 'enforce-topic',
+               default=False,
+    )
+    configitem('experimental', 'topic-mode',
+               default=None,
+    )
+    configitem('_internal', 'keep-topic',
+               default=False,
+    )
+
+    def extsetup(ui):
+        # register config that strickly belong to other code (thg, core, etc)
+        #
+        # To ensure all config items we used are registerd, we register them if
+        # nobody else did so far.
+        from mercurial import configitems
+        extraitem = functools.partial(configitems._register, ui._knownconfig)
+        if ('experimental' not in ui._knownconfig
+                or not ui._knownconfig['experimental'].get('thg.displaynames')):
+            extraitem('experimental', 'thg.displaynames',
+                      default=None,
+            )
+        if ('devel' not in ui._knownconfig
+                or not ui._knownconfig['devel'].get('random')):
+            extraitem('devel', 'randomseed',
+                      default=None,
+            )
+
 def _contexttopic(self, force=False):
     if not (force or self.mutable()):
         return ''
@@ -249,7 +282,7 @@
 
     repo = repo.unfiltered()
 
-    if repo.ui.config('experimental', 'thg.displaynames', None) is None:
+    if repo.ui.config('experimental', 'thg.displaynames') is None:
         repo.ui.setconfig('experimental', 'thg.displaynames', 'topics',
                           source='topic-extension')
 
@@ -928,7 +961,7 @@
 def _configtopicmode(ui):
     """ Parse the config to get the topicmode
     """
-    topicmode = ui.config('experimental', 'topic-mode', default=None)
+    topicmode = ui.config('experimental', 'topic-mode')
 
     # Fallback to read enforce-topic
     if topicmode is None: