This problem appeared with the last EAP (142.4859.7)
When you split the main editor window horizontaly and then trying to change the size of the window you begging to dragging tabs, what is really annoying. Especially if you're dragging border up, in the case the tab jumps to the upper window.
In the version EAP 142.4245.2 such behaviour isnt presented.
OS: Ubuntu 14.04