A 3-year-old boy is brought to the ophthalmology clinic by his mother with a complaint of inward turning of the right eye since age 2 years. On examination, the right eye is deviated medially at rest. The cover test shows that when the right eye is covered, the left eye makes an outward movement to fixate. When the left eye is covered, the right eye makes an inward movement to fixate. Both eyes have full extraocular movements. Visual acuity is normal in both eyes. What is the most likely diagnosis?