I remember something like this with a friend’s water bed. It was mostly a normal king size bed setup but the rim of the bed had a 3 or 4 inch wide padded lip. It all came out to level when the bed was made but the effect as you got on the bed was sinking down a little past the lip, so even if you were on the edge you were still sort of cupped into the mattress instead of being able to easily roll out.
I remember something like this with a friend’s water bed. It was mostly a normal king size bed setup but the rim of the bed had a 3 or 4 inch wide padded lip. It all came out to level when the bed was made but the effect as you got on the bed was sinking down a little past the lip, so even if you were on the edge you were still sort of cupped into the mattress instead of being able to easily roll out.