diff -r 01cf426bd458 -r 6275808e89ef hgext3rd/topic/__init__.py
--- a/hgext3rd/topic/__init__.py	Thu Sep 14 20:33:03 2017 +0200
+++ b/hgext3rd/topic/__init__.py	Wed Sep 13 15:13:51 2017 +0200
@@ -58,6 +58,7 @@
 from mercurial.i18n import _
 from mercurial import (
     bookmarks,
+    changelog,
     cmdutil,
     commands,
     context,
@@ -307,6 +308,26 @@
         repo.names.addnamespace(namespaces.namespace(
             'topics', 'topic', namemap=_namemap, nodemap=_nodemap,
             listnames=lambda repo: repo.topics))
+    # Wrap workingctx extra to return the topic name
+    extensions.wrapfunction(context.workingctx, '__init__', wrapinit)
+    # Wrap changelog.add to drop empty topic
+    extensions.wrapfunction(changelog.changelog, 'add', wrapadd)
+
+def wrapinit(orig, self, repo, *args, **kwargs):
+    orig(self, repo, *args, **kwargs)
+    if repo.currenttopic:
+        self._extra[constants.extrakey] = repo.currenttopic
+    else:
+        # Empty key will be dropped from extra by another hack at the changegroup level
+        self._extra[constants.extrakey] = ''
+
+def wrapadd(orig, cl, manifest, files, desc, transaction, p1, p2, user,
+            date=None, extra=None):
+    if constants.extrakey in extra and not extra[constants.extrakey]:
+        extra = extra.copy()
+        del extra[constants.extrakey]
+    return orig(cl, manifest, files, desc, transaction, p1, p2, user,
+                date=date, extra=extra)
 
 # revset predicates are automatically registered at loading via this symbol
 revsetpredicate = topicrevset.revsetpredicate