diff --git a/hgext/pager.py b/hgext/pager.py
--- a/hgext/pager.py
+++ b/hgext/pager.py
@@ -87,14 +87,10 @@ def _runpager(ui, p):
                              close_fds=util.closefds, stdin=subprocess.PIPE,
                              stdout=util.stdout, stderr=util.stderr)
 
-    # back up original file objects and descriptors
-    olduifout = ui.fout
-    oldstdout = util.stdout
+    # back up original file descriptors
     stdoutfd = os.dup(util.stdout.fileno())
     stderrfd = os.dup(util.stderr.fileno())
 
-    # create new line-buffered stdout so that output can show up immediately
-    ui.fout = util.stdout = newstdout = os.fdopen(util.stdout.fileno(), 'wb', 1)
     os.dup2(pager.stdin.fileno(), util.stdout.fileno())
     if ui._isatty(util.stderr):
         os.dup2(pager.stdin.fileno(), util.stderr.fileno())
@@ -103,15 +99,10 @@ def _runpager(ui, p):
     def killpager():
         if util.safehasattr(signal, "SIGINT"):
             signal.signal(signal.SIGINT, signal.SIG_IGN)
-        pager.stdin.close()
-        ui.fout = olduifout
-        util.stdout = oldstdout
-        # close new stdout while it's associated with pager; otherwise stdout
-        # fd would be closed when newstdout is deleted
-        newstdout.close()
-        # restore original fds: stdout is open again
+        # restore original fds, closing pager.stdin copies in the process
         os.dup2(stdoutfd, util.stdout.fileno())
         os.dup2(stderrfd, util.stderr.fileno())
+        pager.stdin.close()
         pager.wait()
 
 def uisetup(ui):
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -63,9 +63,21 @@ urlparse = pycompat.urlparse
 urlreq = pycompat.urlreq
 xmlrpclib = pycompat.xmlrpclib
 
+def isatty(fp):
+    try:
+        return fp.isatty()
+    except AttributeError:
+        return False
+
+# glibc determines buffering on first write to stdout - if we replace a TTY
+# destined stdout with a pipe destined stdout (e.g. pager), we want line
+# buffering
+if isatty(stdout):
+    stdout = os.fdopen(stdout.fileno(), 'wb', 1)
+
 if pycompat.osname == 'nt':
     from . import windows as platform
-    stdout = platform.winstdout(pycompat.stdout)
+    stdout = platform.winstdout(stdout)
 else:
     from . import posix as platform
 
@@ -2750,12 +2762,6 @@ def removeauth(u):
     u.user = u.passwd = None
     return str(u)
 
-def isatty(fp):
-    try:
-        return fp.isatty()
-    except AttributeError:
-        return False
-
 timecount = unitcountfn(
     (1, 1e3, _('%.0f s')),
     (100, 1, _('%.1f s')),