I don't like the way the pages look when the headers shrink. I kind of feel naked without it. I am used to seeing my avatar in the top left. It also removes the Watchlist, a feature I often use to check for new replies in my own threads and posts in discussions I have previously participated in that are of interest to me.
It's not about like or dislike, this is very useful feature for people who are using devices with smaller screen size, like smartphones and tablets, since we don't have responsive mobile theme yet.
And btw in other smf forums this shrink/expand feature is working just fine by default.