Google led me to a Gentoo site where ebichu was having the same issue, and posted the solution. I can't thank him there since I don't have a gentoo.org password, but he or she deserves thanks. The fix is to add a line to /etc/gtk-3.0/settings.ini:
gtk-primary-button-warps-slider = false
So they apparently call this slider warping. Very user-hostile.