webmail/lib/email-body-selection.ts

import { Email } from '@/lib/jmap/types';
import { hasMeaningfulHtmlBody } from '@/lib/signature-utils';

/**
 * Pick the HTML body an email should be rendered from, or null when it should be
 * rendered as plain text instead. Shared by the desktop viewer and the mobile
 * thread view so the two cannot drift apart again (#489: the thread view was
 * missing the part-type guard and rendered plain-text-only mail as HTML,
 * collapsing every line break).
 */
export function getRenderableHtmlBody(
  email: Pick<Email, 'htmlBody' | 'textBody' | 'bodyValues'>
): string | null {
  const htmlPart = email.htmlBody?.[0];
  if (!htmlPart?.partId || !email.bodyValues?.[htmlPart.partId]) return null;

  const htmlContent = email.bodyValues[htmlPart.partId].value;
  if (!htmlContent) return null;

  // Per RFC 8621, a single-part email exposes the same part in both htmlBody and
  // textBody. That shared part may actually be text/plain (plain-text-only mail);
  // rendering it as HTML collapses newlines and skips linkification, so route by
  // the part's media type rather than by which list it appears in.
  if (htmlPart.type && htmlPart.type.toLowerCase() !== 'text/html') return null;

  // Prefer textBody when the HTML is an auto-generated minimal wrapper (no rich
  // formatting). Server-generated HTML from text/plain emails often lacks <br>
  // tags, which would collapse newlines just the same.
  const textPartId = email.textBody?.[0]?.partId;
  const hasDistinctTextBody =
    !!textPartId && textPartId !== htmlPart.partId && !!email.bodyValues[textPartId];
  if (hasDistinctTextBody && !hasMeaningfulHtmlBody(htmlContent)) return null;

  return htmlContent;
}