Tab stacks are a tab-grouping mechanism similar in concept to Firefox's Panorama, but completely based in the tab bar. To use it, drag one tab on top of another. The bottom tab will disappear, and an arrow will appear to the right of the tab. Click it and the tabs in the stack will slide out to one side. To break up a stack, drag a tab off the stack. Mouse over the stack to see previews for all the tabs in the stack.