pinafore/routes/_components/compose/ComposeLengthIndicator.html

<span class="compose-box-length {{textOverLimit ? 'over-char-limit' : ''}}"
      aria-label="{{lengthLabel}}">
      {{lengthToDisplayAfterRaf || '0'}}
    </span>
<style>
  .compose-box-length {
    grid-area: length;
    justify-self: right;
    color: var(--main-theme-color);
    font-size: 1.3em;
    align-self: center;
  }

  .compose-box-length.over-char-limit {
    color: var(--warning-color);
  }
</style>
<script>
  import { CHAR_LIMIT } from '../../_static/statuses'
  import { mark, stop } from '../../_utils/marks'
  import { store } from '../../_store/store'

  export default {
    oncreate() {
      // perf improvement for keyboard input latency
      this.observe('lengthToDisplay', lengthToDisplay => {
        requestAnimationFrame(() => {
          mark('set lengthToDisplayAfterRaf')
          this.set({lengthToDisplayAfterRaf: lengthToDisplay})
          stop('set lengthToDisplayAfterRaf')
        })
      })
    },
    store: () => store,
    computed: {
      lengthToDisplay: (textLength) => {
        return CHAR_LIMIT - textLength
      },
      lengthLabel: (textOverLimit, lengthToDisplay) => {
        if (textOverLimit) {
          return `${lengthToDisplay} characters over limit`
        } else {
          return `${lengthToDisplay} characters remaining`
        }
      }
    }
  }
</script>