foldtext.vim

function! MyFoldText() " {{{
    let line = getline(v:foldstart)

    " so that it works when relativenumber has been set
    if (&relativenumber)
        let nucolwidth = &fdc + &relativenumber * &numberwidth
    else
        let nucolwidth = &fdc + &number * &numberwidth
    endif

    let windowwidth = winwidth(0) - nucolwidth - 3
    let foldedlinecount = v:foldend - v:foldstart

    " expand tabs into spaces
    let onetab = strpart('          ', 0, &tabstop)
    let line = substitute(line, '\t', onetab, 'g')

    let line = strpart(line, 0, windowwidth - 2 -len(foldedlinecount))
    let fillcharcount = windowwidth - len(line) - len(foldedlinecount)
    return line . '…' . repeat(" ",fillcharcount) . foldedlinecount . '…' . ' '
endfunction " }}}
set foldtext=MyFoldText()

foldtextexample.markdown