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
648c4689273647c321dd6e3979d910282e9a9339.
.resizable(@direction) {
resize: @direction; // Options: horizontal, vertical, both
- overflow: auto; // Safari fix
+ overflow: auto; // Per CSS3 UI, `resize` only applies when `overflow` isn't `visible`
}