russelldavis/Gmail-Preview-Pane-Arrow-Scrolling-Userscript
This script simply restores the ability to scroll the preview pane in Gmail with the arrow keys on the keyboard. Gmail changed the behavior of the arrow keys to move up and down in the thread list, leaving no way to scroll the preview pane except by paging up/down. If you do not use the Preview Pane lab in Gmail, this script is not for you.