diff -r b27cf9f52194 -r aaa046919043 mercurial/ui.py
--- a/mercurial/ui.py	Fri Nov 01 21:46:34 2019 -0700
+++ b/mercurial/ui.py	Sat Nov 02 15:33:39 2019 -0700
@@ -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