I have wanted to use the tmux integration in iTerm2 for a long time now, but whenever I would go in to iTerm2, it would change things so that the mouse wheel (trackpad scroll) would cycle through the shell history instead of scrolling back through the buffer. I cannot image any sane person would want it to work that way, but there you go.
I finally figured out how to set it back to normal scrollback behavior. There is an advanced setting in iTerm2 buried way down in the advanced settings:
iTerm2 –> Settings –> Advanced (icon) –> Scroll down about half-way through to the “Mouse” section –> Change “Scroll wheel sends arrow keys when in alternate screen mode.” from Yes to No.
Poof. Problem solved.