A 28-year-old woman presents to the ophthalmology outpatient department with a 6-month history of intermittent horizontal diplopia that worsens toward the end of the day and with prolonged near work. She reports that her right eye drifts outward when she is tired, but the deviation is not constant. On examination, the visual acuity is 6/6 in both eyes. The cover test shows that when the right eye is covered, the left eye makes a small inward movement to take up fixation (phoria). When the right eye is uncovered, it quickly realigns. Ocular motility is full in all gazes. There is no ptosis or pupillary involvement. What is the most likely diagnosis?