As far as I can determine, they used it every time window was resized to get current viewport and update `width:...;height:...;` CSS styling. I updated mask element to use `left:0;right:0;top:0;bottom:0;` "stretching" technique that doesn't need it.

Because it was exposed as part of public API, I kept it in case we used it somewhere (better to get a meaningful exception than an `undefined` error).