A black bar appeared at the bottom of the new window after I drag a tab out.

this is actually an issue back in the mavericks safari, but the problem was somehow solvable and was not appearing every time.

after upgrading to Yosemite, this problem appears every time.

The problem is : when I have several tabs, then I drag a tab out, which you can see the minimized window.

After yourelease it, it will become a new window as expected and intended. BUT there will be a black bar underneath.

In the Youtube website, you can see that the upper part of the site is actually being blocked by the toolbar.

( I had waited for several seconds to let the toolbar to get to its original position )

Clearly the black bar at the bottom caused this problem.

It's a very annoying issue.

I am not sure if it's a problem of Yosemite or Safari. But back in to mavericks, the problem also existed.

Is there anyone out there having the same problem?

How to Fix it ?

P.S.1 HOWEVER, if i right-clicked the tab and select "move tab to a new window", the problem DOES NOT exist.

P.S.2 I am using Mac Book Air(13", Mid 2013), OS X Yosemite (10.10.2)