A 35-year-old woman with sudden-onset horizontal diplopia and rightward gaze limitation is found to have esotropia that increases when looking to the right. A 9-year-old boy with lifelong inward deviation of the left eye has a constant esotropia of 25 prism diopters in all gazes. Which clinical finding best distinguishes the paralytic strabismus in the woman from the concomitant strabismus in the boy?