I see no issue here.

As awful as it looks, I think it's stable - assuming the fence is properly secured and the ladder is sturdy. A bigger factor is that it has no hand rails and it's easy to lose balance.

It's not too far from the recommended method for that scenario: https://www.wikihow.com/Paint-a-High-Ceiling-over-Stairs

Although I'd definitely go for a flat surface instead

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