28. The non-transitory computer readable storage medium of claim 21 , the one or more programs further comprising instructions, which when executed by the portable multifunction device, cause the device to: in response to determining that the initial portion of the gesture is detected at a location corresponding to a second region of the electronic document, select a second dynamic disambiguation threshold used to determine whether to perform navigation or annotation.
21. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by a portable multifunction device with a touch screen display, cause the device to: display at least a portion of an electronic document that includes a first region; detect a gesture on the touch-sensitive surface, wherein the gesture comprises an initial portion comprising touchdown of a contact on the touch-sensitive surface, and a subsequent portion after the touchdown; determine whether the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document; in response to determining that the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document, select a first dynamic disambiguation threshold used to determine whether to perform navigation or annotation; in response to detecting movement of the contact: in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture satisfies the first dynamic disambiguation threshold, perform a navigation operation on the electronic document; and in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture does not satisfy the first dynamic disambiguation threshold, perform an annotation operation on the electronic document.