vim-patch:7.4.305

Problem:    Making 'ttymouse' empty after the xterm version was requested
            causes problems. (Elijah Griffin)
Solution:   Do not check for DEC mouse sequences when the xterm version was
            requested.  Also don't request the xterm version when DEC mouse
            was enabled.

https://code.google.com/p/vim/source/detail?r=v7-4-305