Vim: highlight word wrap column in insert mode

I like vim’s colorcolumn for highlighting where word wrap will occur, but I consider it a distraction when I’m not in insert mode. After some tinkering, I wrote this in my .vimrc:

" highlight textwidth column in insert mode
highlight ColorColumn ctermbg=0*
function! HighlightOn()
  if &textwidth > 0
    " the +1 feature in the 'colorcolumn' docs doesn't work for me
    let &colorcolumn=&textwidth + 1
  else
    let &colorcolumn=""
  endif
endfunction
autocmd InsertEnter * :call HighlightOn()
autocmd InsertLeave * let &colorcolumn=""

That note about +1 is me working around a bug. I should be able to just write:

" highlight textwidth column in insert mode highlight ColorColumn ctermbg=0* autocmd InsertEnter * let &colorcolumn=+1 autocmd InsertLeave * let &colorcolumn=""

Unfortunately, some tweak or plugin breaks this feature for me. I wrote this workaround rather than diagnose and fix it properly because the process just seemed too tedious.

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