How to add resizer to textarea in IE?

Internet Explorer (and Microsoft Edge, as of May 5, 2016) do not support native gripper/resizing on textarea elements. While these may eventually be supported in the future (in Microsoft Edge), the best option you have for now is to polyfill the functionality.

Many options exist online that do this, but if you're using jQuery and jQuery UI, you could use the Resizable widget:

$("textarea").resizable({
    handles: "se" // place handle only in 'south-east' of textarea
});

By default, this places the handler beneath the textarea. I didn't like this:

.ui-resizable-handle {
    transform: translateY(-100%);
}

You can see the end-result here: http://jsbin.com/rumokazepo/edit?html,css,js,output