I use IPython 2.3.1 on OS X 10.9.5, RHEL6.5, and Cygwin. When there is a long list of completion candidates, they are arranged to fit on in the terminal. If the terminal is resized, the width it uses to arrange the candidates is not updated.

Completion on a fresh start of IPython:

Completion after resizing the terminal and restarting the completion (canceling or sending the previous command and typing np.<Tab> on a new line to try to refresh the display):

Oddly enough, the completion candidates are displayed correctly after resizing the terminal in Cygwin, but not in OSX or RHEL6.5. I am using the same terminal emulator (mintty) for both Cygwin and RHEL6.5 (via SSH) so I don't think mintty is the problem. I see the same thing in the OSX Terminal application as well as in iTerm2. I also see the same problem with or without readline enabled.