Preview a Markdown draft

const input = api.input() ?? {};
const ui = input.$widget ?? {};
const targets = ui.targetIds ?? ['*'];
const wanted = (id) => targets.includes('*') || targets.includes(id);
const empty = {format: 'markdown', body: '', attachments: []};
const draft = ui.values?.reply ?? empty;
if (!draft || draft.format !== 'markdown' || typeof draft.body !== 'string') {
  throw new Error('Expected a Markdown message draft');
}
const rows = [];
if (wanted('replyForm')) {
  rows.push([{
    id: 'replyForm', type: 'FORM', label: 'Public reply',
    submitLabel: 'Prepare preview',
    fields: [{
      id: 'reply', type: 'INPUT', control: 'message', label: 'Message',
      value: empty, valueMode: 'initial', height: 240, maxLength: 12000,
      ...(typeof input.uploadFolderId === 'string'
        ? {composer: {uploadFolderId: input.uploadFolderId}}
        : {})
    }],
    on: {submit: {type: 'REFRESH', target: ['markdownPreview']}}
  }]);
}
if (wanted('markdownPreview')) {
  rows.push([{
    id: 'markdownPreview', type: 'TEXT', label: 'Committed message', height: 360,
    presentation: {kind: 'markdown', copy: true, fullscreen: true},
    value: draft.body
  }]);
}
return rows;