Iframe is blocking resizer

[Matoo125]

Hello here http://matejvrzala.xyz/projects/Playground/
I am using resizerjs to make two divs resizable (code and output) both are inside container.

<div class="container">
    <div class = "code"> ... </div>
    <div class="output"> <iframe></iframe </div>
</div>

I can perfectly resize to the left (code side).

The problem appears, when I try to resize to the right (iframe output side). If I will go very slowly to the right it is possible to resize, but i cannot move faster than rz-handle. That is not very usable in this case.

Do you have any idea How to make it resize fluently to the right (iframe) size?