vue-select with flex-container

Question

I'm trying to use v-select in admin panel in header (using CoreUI). I have a container with "display: flex" https://codepen.io/kat15pl/pen/NWNZgQg

<div id="app" class="c-app">
  <div class="c-wrapper">
    <header class="c-header c-header-light c-header-fixed c-header-with-subheader">
      <ul class="c-header-nav d-md-down-none mr-auto">
        <li style="width: 100%;">
          <v-select :options="options"/>
        </li>
        <li>
          next div
        </li>
      </ul>
    </header>
  </div>
</div>

I think I do something wrong... but I don't see what:( I want to not resize when select is opened. I mean why input cannot be 100%?