Not a Safari bug. It's standards-compliant.

Quoting from http://www.w3.org/TR/css3-ui/#resize :
> ### 8.1. `resize` property
>
> * Applies to: elements with `overflow` other than `visible`
>
> The `resize` property applies to elements whose computed `overflow` value is something other than `visible`.

Original comment was added in commit 648c4689.