diff --git a/mercurial/ui.py b/mercurial/ui.py
index b27cf9f521943561d15623f3627dd48532f00740_bWVyY3VyaWFsL3VpLnB5..aaa046919043f77b17d0bb1504723ac9a56ccfc3_bWVyY3VyaWFsL3VpLnB5 100644
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -1564,6 +1564,7 @@
         # - http://bugs.python.org/issue12833
         with self.timeblockedsection(b'stdio'):
             if usereadline:
+                self.flush()
                 prompt = encoding.strfromlocal(prompt)
                 line = encoding.strtolocal(pycompat.rawinput(prompt))
                 # When stdin is in binary mode on Windows, it can cause