A computing device with a display simultaneously displays a plurality of user interface objects, a currently selected user interface object, and a plurality of resizing handles for the currently selected user interface object. Each respective resizing handle has a corresponding handle activation region with a default position relative to the respective resizing handle, a default size, and a default shape. The device detects a first input on a first handle activation region for a first resizing handle in the plurality of resizing handles. In response to detecting the first input, the device: resizes the currently selected user interface object, and for at least one resizing handle in the plurality of resizing handles, modifies a corresponding handle activation region by changing the position of the handle activation region relative to the resizing handle from the default position to a modified position.