--- a/hgext3rd/topic/__init__.py	Tue Aug 16 15:43:06 2016 +0200
+++ b/hgext3rd/topic/__init__.py	Tue Aug 23 23:38:28 2016 +0200
@@ -301,6 +301,10 @@
             # XXX we should include the data even when not verbose
             data = stack.stackdata(repo, topic)
             fm.plain(' (')
+            fm.write('branches+', 'on branch: %s',
+                     '+'.join(data['branches']), # XXX use list directly after 4.0 is released
+                     label='topic.list.branches')
+            fm.plain(', ')
             fm.write('changesetcount', '%d changesets', data['changesetcount'],
                      label='topic.list.changesetcount')
             if data['troubledcount']:

--- a/hgext3rd/topic/stack.py	Tue Aug 16 15:43:06 2016 +0200
+++ b/hgext3rd/topic/stack.py	Tue Aug 23 23:38:28 2016 +0200
@@ -91,6 +91,7 @@
             data['behindcount'] = 0
         except error.ManyMergeDestAbort:
             data['behindcount'] = -1
+    data['branches'] = sorted(set(repo[r].branch() for r in revs))
 
     return data
 

--- a/tests/test-topic-list.t	Tue Aug 16 15:43:06 2016 +0200
+++ b/tests/test-topic-list.t	Tue Aug 23 23:38:28 2016 +0200
@@ -173,10 +173,10 @@
 verbose
 
   $ hg topic --verbose
-     bar (5 changesets, 1 troubled, 2 heads)
-     baz (2 changesets, 2 behind)
-   * foo (2 changesets, ambiguous destination)
-     fuz (3 changesets, 2 troubled, 1 behind)
+     bar (on branch: default, 5 changesets, 1 troubled, 2 heads)
+     baz (on branch: default, 2 changesets, 2 behind)
+   * foo (on branch: lake, 2 changesets, ambiguous destination)
+     fuz (on branch: default, 3 changesets, 2 troubled, 1 behind)
 
 json
 
@@ -206,6 +206,7 @@
   [
    {
     "active": false,
+    "branches+": "default",
     "changesetcount": 5,
     "headcount": 2,
     "topic": "bar",
@@ -214,18 +215,21 @@
    {
     "active": false,
     "behindcount": 2,
+    "branches+": "default",
     "changesetcount": 2,
     "topic": "baz"
    },
    {
     "active": true,
     "behinderror": "ambiguous destination",
+    "branches+": "lake",
     "changesetcount": 2,
     "topic": "foo"
    },
    {
     "active": false,
     "behindcount": 1,
+    "branches+": "default",
     "changesetcount": 3,
     "topic": "fuz",
     "troubledcount": 2