vim-patch:7.4.309

Problem:    When increasing the size of the lower window, the upper
            window jumps back to the top. (Ron Aaron)
Solution:   Change setting the topline. (Nobuhiro Takasaki)

https://code.google.com/p/vim/source/detail?r=88a6e9f33822d33b6c32db578750c6c178c63f50