When I use Rubik(6.x-3.0-beta2) admin theme (which implement Tao) in RTL mode, I get horizontal scroll.
I found its because of this lines of CSS in base.css file:

#skipnav {
    left: -1000px; /*<------- this part of style cause this problem;*/
    position: absolute;
    top: -1000px;
}

Comments

#skipnav {
..
left: 0px;
font-size:0px;
...

It Worked !!