When opening up nano when using the solarized terminal theme the screen refresh doesn't seem to work. It also sometimes starts the first character of a line on the end of the previous line. Which always happens on the first line of the file. I have tested this and know its the solarized theme as I have changed themes and it doesn't happen on that theme.
Also when opening a file with lots of lines when I scroll down it sometimes doesn't refresh the page as blank so when it redraws the next page it shows the info from that page instead of some from the previous.
I am also using zsh. If you need any other info please let me know.
I've just come across the same problem. I resolved it by going to the Advanced tab and changing Declare terminal as from ansi to xterm-256color.