Prevention of pressing of <C-P> again if the popup menu is already visible, and user <BS> characters then continue typing again.

.vimrc

" Add this to your vimrc to get a minimalist autocomplete pop
" Or use as a plugin : https://github.com/maxboisvert/vim-simple-complete

" Minimalist-TabComplete-Plugin
inoremap <expr> <Tab> TabComplete()
fun! TabComplete()
    if getline('.')[col('.') - 2] =~ '\K' || pumvisible()
        return "\<C-P>"
    else
        return "\<Tab>"
    endif
endfun

" Minimalist-AutoCompletePop-Plugin
set completeopt=menu,menuone,noinsert
inoremap <expr> <CR> pumvisible() ? "\<C-Y>" : "\<CR>"
autocmd InsertCharPre * call AutoComplete()
fun! AutoComplete()
    if v:char =~ '\K'
        \ && getline('.')[col('.') - 4] !~ '\K'
        \ && getline('.')[col('.') - 3] =~ '\K'
        \ && getline('.')[col('.') - 2] =~ '\K' " last char
        \ && getline('.')[col('.') - 1] !~ '\K'

        if pumvisible()==0
            call feedkeys("\<C-P>", 'n')
        endif
    end
endfun