It would seem that the "actions performed" scroll window under the Details tab of the "Perform Installation" screen in the installer is not scrollable after a certain point. Once the packages are all installed and it is doing the follow-on setup tasks like indexing fonts, setting up the boot-loader, and setting up the next install stage, the screen no longer is scrollable.

This is best exemplified by letting the installer get to that screen and selecting "Details", to watch the install. Once the reboot pop-up prompts, slick Skip. At this point the scroll bar will be unmovable. It was unmovable before this point as well.