On June 2, 2008, the WebKit development team announced SquirrelFish, a new JavaScript engine that vastly improves Safari's speed at interpreting scripts. The engine is one of the new features in Safari 4, released to developers on June 11, 2008. The new JavaScript engine quickly evolved into SquirrelFish Extreme, featuring even further improved performance over SquirrelFish, and was eventually marketed as Nitro. A public beta of Safari 4 was released on February 24, 2009, with new features such as the Top Sites tool (similar to Opera's Speed Dial feature), which displays the user's most visited sites on a 3D wall. Cover Flow, a feature of Mac OS X and iTunes, was also implemented in Safari. In the public beta versions, tabs were placed in the title bar of the window, similar to Google Chrome. The tab bar was moved back to its original location, below the URL bar, in the final release. The Windows version adopted a native Windows theme, rather than the previously employed Mac OS X-style interface. Also, Apple removed the blue progress bar located in the address bar (later reinstated in Safari 5). Safari 4.0.1 was released for Mac on June 17, 2009, and fixed problems with Faces in iPhoto '09. Safari 4 in Mac OS X v10.6 "Snow Leopard" has 64-bit support, which can make JavaScript loading up to 50% faster. It also has built-in crash resistance unique to Snow Leopard; crash resistance will keep the browser intact if a plug-in like Flash player crashes, such that the other tabs or windows will be unaffected. Safari 4.0.4, released on November 11, 2009, for both OS X and Windows, further improves JavaScript performance.