Update: Google has pulled the feature from the Chrome browser again. It is no longer available, the flag has been removed and there is no setting that enables tab stacking in the browser.
I'm not really sure when Google introduced the tab stacking feature in the Google Chrome browser, only that it is available at least in the Windows Chrome Canary builds right now. Tab stacking basically allows you to stack tabs on top of each other to save screen estate when the tab count reaches a level where the browser would originally resort to scrolling.
If I'm not mistaken, it was Opera that introduced tab stacks in version 11 before any other browser. I personally think that all major browsers will eventually introduce a tab stacking feature eventually as it offers a great way of saving screen estate in the tab bar if multiple pages on the same domain name are open in the Internet browser.
Back to Google Chrome and the browser's tab stacking implementation. The feature is currently only available via the chrome://flags list of experimental features. Just load chrome://flags/ in the browser's address bar, hit F3 and enter stacked tabs in the on-page search form to find the parameter immediately on the page.
It reads: Stacked Tabs. Tabs never shrink, instead they stack on top of each other when there is not enough space.
Click on the Enable link and restart the browser to enable tab stacking in Google Chrome. If you have tried Opera's tab stacking functionality before, you will notice that Chrome's differs in core aspects.
In Opera, you simply drag and drop tabs on each other to create a stack. This stack is then visualized with an arrow on the right side of the stack that is pointing away from it. A click on the arrow or a double-click on the active tab displays all tabs of that stack in the tabbar.
In Google Chrome, the tab stacking feature is automatic. When there is not enough room, pages are stacked on top of each other. The method is confusing at first, as the original tab and the newly opened tab are displayed next to each other first, and only added to the stack once you click on an unrelated tab.
Another difference is that you can't drag and drop tabs on top of each other in Chrome.
To paraphrase: Opera supports manual tab stacking while Google Chrome only the automatic creation of tab stacks. It needs to be noted at this point that the Chrome implementation is experimental right now, and that the Chrome devs may make changes to the feature in the future.