vimrc relative numbering. Leaves relative numbering on loss of focus and on entering insert mode. Re-enters when appropriate.

gistfile1.vim

set relativenumber

au InsertEnter * :set number
au InsertLeave * :set relativenumber

function! FocusGainNumbering()
  if(mode() == 'i')
    :set number
  else
    :set relativenumber
  endif
endfunc

au FocusLost * :set number
au FocusGained * :call FocusGainNumbering()