the home screen was also added, though the feature was notably absent from iPhone 3G and the second-generation iPod Touch due to poor performance of icon animations. Multitasking iOS 4 introduced multitasking. The feature allowed users to switch between apps instantly by double-clicking the home button. It was implemented in such a way that did not cause excessive battery drain. Multitasking was limited to apps dealing with Internet calling, location, and audio playback, while a similar "Fast App Switching" technology meant users could leave an app and enter another, with the original app remaining in the background until the user returns. This feature was notably absent from iPhone 3G and the second-generation iPod Touch due to performance issues. Spell check iOS 4 introduced a spell checking feature that underlined misspelled words in red. Tapping on the word would provide a pop-up with a recommended replacement. Camera The Camera app now supports 5x digital zoom. App features iBooks iOS 4 introduced iPhone and iPod Touch support in iBooks, which was already included on iPad. Though not a default app, it was available through App Store. Mail The Mail app featured a unified inbox on iOS 4, allowing users to see messages from all of their email accounts displayed together in a single inbox. It also gained support for MobileMe e-mail aliases and multiple Exchange accounts for business users. Game Center iOS 4.1 added a new app called Game Center, an online multiplayer social gaming network, which allows users to invite friends to play games and to compare their scores on a leaderboard. It was not available on the iPhone 3G. FaceTime iOS 4 introduced FaceTime, a videotelephony app that uses the device's camera to allow the user to make video calls with other FaceTime users. This feature was absent from the iPhone 3G, second-generation iPod Touch, iPhone 3GS, and third-generation iPod Touch due to the lack of required features, such as a front-facing camera. Safari The Safari mobile web browser on iOS 4 added Bing as a search option in addition to Google and Yahoo!. Beginning with iOS 4.2.1, specific words or phrases on a page could be searched. Problems Performance and battery issues iPhone 3G users reported performance and battery issues after upgrading to iOS 4. Apple started an investigation of the matter in July 2010. In November, Apple was sued for the issues, with an unsatisfied customer alleging "violating