pinafore/routes/_components/compose/ComposeButton.html

<button class="primary compose-box-button"
        :disabled >
  Toot!
</button>
<style>
  .compose-box-button {
    grid-area: button;
    justify-self: right;
    text-transform: uppercase;
    margin-top: 10px;
  }
</style>
<script>
  import { store } from '../../_store/store'

  export default {
    store: () => store,
    computed: {
      disabled: (textOverLimit, textLength) => {
        return textOverLimit || textLength === 0
      }
    }
  }
</script>