vim-patch:7.4.281  #746

Problem:    When a session file has more than one tabpage and 'showtabline' is
            one the positions may be slightly off.
Solution:   Set 'showtabline' to two while positioning windows.

https://code.google.com/p/vim/source/detail?r=24c90f1fec859b54cf2b854b98c4c9e614c46061