There's a lot of confusion documented in this bug https://bugs.chromium.org/p/chromium/issues/detail?id=99364 about how border-radius affects clipping. (It curves its own box edges, but it doesn't cause an element with overflow: hidden to start clipping its descendants to its content edge instead of its padding edge.)

The use case could be easily handled by adding keywords like content-box/padding-box/border-box to the new overflow-clip-margin property, though. They would set the zero position of the offset to that edge.

CC @laukstein