This is the default action since 5.1. From the iOS 6 release notes:
In iOS 5.1, the UISplitViewController class adopts a sliding presentation style when presenting the left view (previously seen only in Mail). This style is used when a presentation is initiated either by an existing panel button element provided by delegate methods or by swiping the screen in the right direction. No additional API implementation is required to get this behavior, and all existing APIs, including the UIPopoverController instance provided by the delegate, continue to work.
If the gesture is not acceptable in your application, set the Represents property of theithGesture of your split view controller to NO to disable the gesture. However, disabling gestures is not recommended since its use provides a consistent user interface in all applications.
Here's the dirty way to fix it.
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