Fixing scrolling in nano running in tmux in mate-terminal

From the tmux FAQ:

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* PLEASE NOTE: most display problems are due to incorrect TERM! Before       *
* reporting problems make SURE that TERM settings are correct inside and     *
* outside tmux.                                                              *
*                                                                            *
* Inside tmux TERM must be "screen" or similar (such as "screen-256color").  *
* Don't bother reporting problems where it isn't!                            *
*                                                                            *
* Outside, it must match your terminal: particularly, use "rxvt" for rxvt    *
* and derivatives.                                                           *
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http://tmux.git.sourceforge.net/git/gitweb.cgi?p=tmux/tmux;a=blob;f=FAQ