I previously reported this bug as No. 53442 and this was determined to be a duplicate of 47506.
I have reviewed 47506 but this is not the same bug. 47506 apparently is intermittent and sometimes moves some icons on the line down.
This bug started in the English Wikipedia where the tab line wraps and overwrites the following line when the screen is reduced to half width. It seemed to correspond with the introduction of the full-screen editor. This originally occurred only in the English version and started only about two months ago (about July 2013). Prior to that it was fine. At first the French wikipedia was unaffected but it now also occurs there and I have also seen it in the German.
It is easy to reproduce - go to the Wikipedia Home page (or any page), reduce the screen width to half width and observe the tab line wrap-around obscuring the title.
Note that this does not only occur when editing but always happens whether editing or not. I use Chrome but it also occurs in Internet Explorer 8.