Scrolling up in iTerm2 shows previous Vim sessions

In iTerm2 (Version 2.1.4), when I scroll up to see previous commands, I'm seeing chunks of previous Vim sessions, which I don't want to see. Any idea why that would be happening and how to fix that?

As an example, here's some output after I typed a line of a file in Vim, then scrolled up. The top line is a remnant of the Vim session. (I have line numbers on in Vim, which is where the 1 comes from.)

  1 I'm typing a line in Vim.
Last login: Tue Jan  5 10:16:18 on ttys001
imac:~$ vi test.txt
imac:~$ cat test.txt
I'm typing a line in Vim.
imac:~$

Sometimes much larger remnants are displayed. Any thoughts would be appreciated.

George Nachman, the iTerm2 guy, ended up solving my problem here. In short, I just needed to uncheck Prefs > Profiles > Terminal > Save lines to scrollback in alternate screen mode in the iTerm2 menu.