Resizing a textarea (with a transition such as transition:all 1s linear) using the handle feels laggy, resulting in undesirable and confusing visual feedback. An example can be seen at http://designshack.net/tutorialexamples/css3-form-transitions/index.html Other browsers also appear to experience this issue. You could obviously avoid it by specifying the CSS properties you want transitioned instead of "all", or apply resize:none. Nevertheless, if one actually wants to use a transition on width/height and enable the handle, this becomes a problem... I can't imagine a scenario where this effect would be desirable.
So the problem is that the resize handle just changes computed width/height, at which point the transition kicks in.... I'm not sure there's a good solution here short of totally changing how resizing works.
And that's a CSS spec issue, not a form control issue.