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Thank you for your post!
What you are describing is the medical term, Nystagmus:
There are certain breeds of cats, this included, where this is a "normal" trait for some pets.
I personally had a Himalayan that had this - with no other signs of illness.
Certainly, with other signs of illness such as falling over or other neurological signs - that would not be normal - but if there are no other signs of illness and this is a constant movement that you have noticed throughout Seashell's entire life, this is not a concern for health or other illness.