diff --git a/hgext/patchbomb.py b/hgext/patchbomb.py
--- a/hgext/patchbomb.py
+++ b/hgext/patchbomb.py
@@ -715,7 +715,6 @@ def email(ui, repo, *revs, **opts):
             m['Reply-To'] = ', '.join(replyto)
         if opts.get('test'):
             ui.status(_('displaying '), subj, ' ...\n')
-            ui.flush()
             ui.pager('email')
             generator = emailmod.Generator.Generator(ui, mangle_from_=False)
             try:
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -845,6 +845,7 @@ class ui(object):
             return
 
         self.debug('starting pager for command %r\n' % command)
+        self.flush()
         self.pageractive = True
         # Preserve the formatted-ness of the UI. This is important
         # because we mess with stdout, which might confuse