title: Is tmux supports cterm=undercurl used in VIM?
body: <p>In my vim config I've added <code>undercurl</code> value for badly spelled words</p>
  
  <pre><code>hi SpellBad guifg=NONE cterm=undercurl
  </code></pre>
  
  <p>Outside the tmux it works fine, but inside of it I continue to see like it is set to <code>underline</code> instead of <code>undercurl</code>.</p>
  
  <p>To clarify that even in vim I couldn't see curly line until I added to my <code>.vimrc</code> this:</p>
  
  <pre><code>let &amp;t_Cs = "\e[4:3m"
  let &amp;t_Ce = "\e[4:0m"
  </code></pre>
  
  <p>I did some long research, it's normal for tmux to behave like this? It seems to me that it doesn't support it at all.</p>
  
  <p><strong>Software conf</strong>:
  <em>Ubuntu 18.04.3 LTS</em>,
  <em>Tmux 3.0a</em>,
  <em>Vim 8.0.1453</em></p>