--- a/CHANGELOG	Sun Dec 02 16:52:14 2018 +0100
+++ b/CHANGELOG	Fri Sep 21 11:44:34 2018 -0400
@@ -8,8 +8,8 @@
   * next: evolve aspiring children by default (use --no-evolve to skip)
   * next: pick lower part of a split as destination
   * compat: drop compatibility with Mercurial 4.3
-  i* topics: improve the message around topic changing
-
+  * topics: improve the message around topic changing
+  * stack: introduce a --children flag (see help for details)
 
 8.3.2 --2017-11-27
 -------------------

--- a/hgext3rd/topic/__init__.py	Sun Dec 02 16:52:14 2018 +0100
+++ b/hgext3rd/topic/__init__.py	Fri Sep 21 11:44:34 2018 -0400
@@ -721,6 +721,8 @@
     return ret
 
 @command('stack', [
+        ('c', 'children', None,
+            _('display data about children outside of the stack'))
     ] + commands.formatteropts,
     _('hg stack [TOPIC]'))
 def cmdstack(ui, repo, topic='', **opts):

--- a/hgext3rd/topic/stack.py	Sun Dec 02 16:52:14 2018 +0100
+++ b/hgext3rd/topic/stack.py	Fri Sep 21 11:44:34 2018 -0400
@@ -329,15 +329,32 @@
 
         symbol = None
         states = []
+        msg = ''
         iscurrentrevision = repo.revs('%d and parents()', ctx.rev())
+        if opts.get('children'):
+            if branch:
+                t_msg = '-branch("%s")' % branch
+            if topic:
+                t_msg = '-topic("%s")' % topic
+            rev_msg = 'children(%s) and merge() %s'
+            revisions = repo.revs(rev_msg % (ctx.rev(), t_msg))
+            len_rev = len(revisions)
+            if len_rev > 0:
+                msg = 'external-children'
 
         if iscurrentrevision:
-            states.append('current')
             symbol = '@'
+            if msg:
+                states.append('current - ' + msg)
+            else:
+                states.append('current')
 
         if ctx.orphan():
             symbol = '$'
-            states.append('unstable')
+            if msg:
+                states.append('unstable - ' + msg)
+            else:
+                states.append('unstable')
 
         if not isentry:
             symbol = '^'
@@ -347,7 +364,10 @@
         # none of the above if statments get executed
         if not symbol:
             symbol = ':'
-            states.append('clean')
+            if msg:
+                states.append(msg)
+            else:
+                states.append('clean')
 
         states.sort()
 

--- a/tests/test-topic-stack.t	Sun Dec 02 16:52:14 2018 +0100
+++ b/tests/test-topic-stack.t	Fri Sep 21 11:44:34 2018 -0400
@@ -229,8 +229,52 @@
   s1: c_c
   s0^ c_b (base)
 
+merge case (displaying info about external)
+-------------------------------------------
+
+  $ hg up default
+  0 files updated, 0 files merged, 4 files removed, 0 files unresolved
+  $ hg topics zzz
+  marked working directory as topic: zzz
+  $ echo zzz > zzz
+  $ hg add zzz
+  $ hg commit -m zzz_a
+  active topic 'zzz' grew its first changeset
+  (see 'hg help topics' for more information)
+  $ hg merge foo
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -m "merged foo"
+
+stack -m display data about child
+
+  $ hg stack foo
+  ### topic: foo
+  ### target: default (branch)
+  s4: c_f
+  s3: c_e
+  s2: c_d
+  s1: c_c
+  s0^ c_b (base)
+
+  $ hg stack foo --children
+  ### topic: foo
+  ### target: default (branch)
+  s4: c_f (external-children)
+  s3: c_e
+  s2: c_d
+  s1: c_c
+  s0^ c_b (base)
+
 error case, nothing to list
 
+  $ hg strip --config extensions.strip= t1 --no-backup
+  0 files updated, 0 files merged, 5 files removed, 0 files unresolved
+
+  $ hg up foo
+  switching to topic foo
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
   $ hg topic --clear
   $ hg stack
   ### target: default (branch)