pinafore/routes/_components/compose/ComposeButton.html

<button class="primary compose-box-button"
        :disabled
        on:click>
  <span class="{{$postingStatus ? 'hidden' : ''}}">
    Toot!
  </span>
  <div class="compose-box-button-spinner {{$postingStatus ? 'spin' : 'hidden'}}"
       aria-hidden="true">
    <svg class="compose-box-button-spinner-svg">
      <use xlink:href="#fa-spinner" />
    </svg>
  </div>
</button>
<style>
  .compose-box-button {
    grid-area: button;
    justify-self: right;
    text-transform: uppercase;
    margin-top: 10px;
    position: relative;
  }
  .compose-box-button-spinner {
    pointer-events: none;
    position: absolute;
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 10;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
  }
  .compose-box-button-spinner-svg {
    width: 24px;
    height: 24px;
    fill: var(--button-primary-text);
  }
</style>
<script>
  import { store } from '../../_store/store'

  export default {
    store: () => store,
    computed: {
      disabled: ($postingStatus, overLimit) => $postingStatus || overLimit
    }
  }
</script>