--- a/hgext3rd/topic/__init__.py	Mon Aug 15 00:19:44 2016 +0200
+++ b/hgext3rd/topic/__init__.py	Sun Aug 14 19:57:58 2016 +0200
@@ -47,6 +47,8 @@
 colortable = {'topic.active': 'green',
               'topic.list.troubledcount': 'red',
               'topic.list.headcount.multiple': 'yellow',
+              'topic.list.behindcount': 'cyan',
+              'topic.list.behinderror': 'red',
               'topic.stack.index': 'yellow',
               'topic.stack.state.base': 'dim',
               'topic.stack.state.clean': 'green',
@@ -307,6 +309,16 @@
                 fm.write('headcount', '%d heads',
                          data['headcount'],
                          label='topic.list.headcount.multiple')
+            if 0 < data['behindcount']:
+                fm.plain(', ')
+                fm.write('behindcount', '%d behind',
+                         data['behindcount'],
+                         label='topic.list.behindcount')
+            elif -1 == data['behindcount']:
+                fm.plain(', ')
+                fm.write('behinderror', '%s',
+                         _('ambiguous destination'),
+                         label='topic.list.behinderror')
             fm.plain(')')
         fm.plain('\n')
     fm.end()

--- a/hgext3rd/topic/stack.py	Mon Aug 15 00:19:44 2016 +0200
+++ b/hgext3rd/topic/stack.py	Sun Aug 14 19:57:58 2016 +0200
@@ -4,6 +4,7 @@
 # GNU General Public License version 2 or any later version.
 from mercurial.i18n import _
 from mercurial import (
+    destutil,
     error,
     node,
 )
@@ -69,6 +70,7 @@
     :changesetcount: number of non-obsolete changesets in the stack
     :troubledcount: number on troubled changesets
     :headcount: number of heads on the topic
+    :behindcount: number of changeset on rebase destination
     """
     data = {}
     revs = repo.revs("topic(%s) - obsolete()", topic)
@@ -76,6 +78,19 @@
     data['troubledcount'] = len([r for r in revs if repo[r].troubled()])
     deps, rdeps = builddependencies(repo, revs)
     data['headcount'] = len([r for r in revs if not rdeps[r]])
+    data['behindcount'] = 0
+    if revs:
+        minroot = [min(r for r in revs if not deps[r])]
+        try:
+            dest = destutil.destmerge(repo, action='rebase',
+                                      sourceset=minroot,
+                                      onheadcheck=False)
+            data['behindcount'] = len(repo.revs("only(%d, %ld)", dest,
+                                                minroot))
+        except error.NoMergeDestAbort:
+            data['behindcount'] = 0
+        except error.ManyMergeDestAbort:
+            data['behindcount'] = -1
 
     return data
 

--- a/tests/test-topic-list.t	Mon Aug 15 00:19:44 2016 +0200
+++ b/tests/test-topic-list.t	Sun Aug 14 19:57:58 2016 +0200
@@ -174,9 +174,9 @@
 
   $ hg topic --verbose
      bar	(5 changesets, 1 troubled, 2 heads)
-     baz	(2 changesets)
-   * foo	(2 changesets)
-     fuz	(3 changesets, 2 troubled)
+     baz	(2 changesets, 2 behind)
+   * foo	(2 changesets, ambiguous destination)
+     fuz	(3 changesets, 2 troubled, 1 behind)
 
 json
 
@@ -213,16 +213,19 @@
    },
    {
     "active": false,
+    "behindcount": 2,
     "changesetcount": 2,
     "topic": "baz"
    },
    {
     "active": true,
+    "behinderror": "ambiguous destination",
     "changesetcount": 2,
     "topic": "foo"
    },
    {
     "active": false,
+    "behindcount": 1,
     "changesetcount": 3,
     "topic": "fuz",
     "troubledcount": 2