From a69b3651fd41bb9d36d9eeab96d7a394deedb4a7 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Mon, 31 Aug 2026 20:41:45 +0200
Subject: [PATCH] fix: render plain-text-only mail as text in the mobile thread
 view #489

---
 components/email/email-viewer.tsx             |  33 +-----
 components/email/thread-conversation-view.tsx |  23 +---
 lib/__tests__/email-body-selection.test.ts    | 107 ++++++++++++++++++
 lib/email-body-selection.ts                   |  35 ++++++
 4 files changed, 149 insertions(+), 49 deletions(-)
 create mode 100644 lib/__tests__/email-body-selection.test.ts
 create mode 100644 lib/email-body-selection.ts

diff --git a/components/email/email-viewer.tsx b/components/email/email-viewer.tsx
index 9af8480d7..8599c3bf2 100644
--- a/components/email/email-viewer.tsx
+++ b/components/email/email-viewer.tsx
@@ -5,7 +5,7 @@ import { Email, ContactCard, Mailbox } from "@/lib/jmap/types";
 import { emailExportFilename, attachmentDownloadFilename, attachmentsBundleFilename, DEFAULT_EMAIL_TEMPLATE, DEFAULT_ATTACHMENT_TEMPLATE } from "@/lib/download-filename";
 import { EML_IMPORT_ACCEPT, expandImportableEmails } from "@/lib/eml-import";
 import { applyNewTabToAnchor, escapeHtml, plainTextToSafeHtml, sanitizeEmailBodyForIframe, sanitizeEmailHtml, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
-import { hasMeaningfulHtmlBody } from "@/lib/signature-utils";
+import { getRenderableHtmlBody } from "@/lib/email-body-selection";
 import { collapsePlainTextQuotes, setupQuoteCollapse } from "@/lib/quote-collapse";
 import { withBasePath } from "@/lib/browser-navigation";
 import { buildContactsPath, buildMailPath } from "@/lib/deep-links";
@@ -1726,35 +1726,10 @@ export function EmailViewer({
 
     // Check if we have body values
     if (email.bodyValues) {
-      // Check if HTML content exists and if it's actually rich HTML or just plain text wrapper
-      let useHtmlVersion = false;
-      let htmlContent = '';
+      // Rich HTML, or just a plain-text wrapper the server dressed up as HTML?
+      let htmlContent = getRenderableHtmlBody(email);
 
-      if (email.htmlBody?.[0]?.partId && email.bodyValues[email.htmlBody[0].partId]) {
-        htmlContent = email.bodyValues[email.htmlBody[0].partId].value;
-        // Per RFC 8621 § 4.1.4, when a message has only one alternative the server
-        // exposes the same part in both htmlBody and textBody. The shared part may
-        // actually be text/plain (plain-text-only mail) - rendering that as HTML
-        // collapses newlines and skips linkification, so route by the part's type.
-        const htmlPart = email.htmlBody[0];
-        if (htmlPart.type && htmlPart.type.toLowerCase() !== 'text/html') {
-          useHtmlVersion = false;
-        } else {
-          // Prefer textBody when HTML is auto-generated minimal wrapper (no rich formatting).
-          // Server-generated HTML from text/plain emails often lacks <br> tags, collapsing newlines.
-          const textPartId = email.textBody?.[0]?.partId;
-          const htmlPartId = htmlPart.partId;
-          const hasDistinctTextBody = !!textPartId && textPartId !== htmlPartId && !!email.bodyValues[textPartId];
-          if (hasDistinctTextBody && htmlContent) {
-            useHtmlVersion = hasMeaningfulHtmlBody(htmlContent);
-          } else {
-            useHtmlVersion = !!htmlContent;
-          }
-        }
-      }
-
-      // If we should use HTML version and it exists
-      if (useHtmlVersion && htmlContent) {
+      if (htmlContent) {
         // Replace cid: references with authenticated blob URLs (fetched via useEffect)
         // This prevents browser auth dialogs that occur when loading raw JMAP download URLs
         if (email.attachments) {
diff --git a/components/email/thread-conversation-view.tsx b/components/email/thread-conversation-view.tsx
index 26de3e08d..d3a1c8ee2 100644
--- a/components/email/thread-conversation-view.tsx
+++ b/components/email/thread-conversation-view.tsx
@@ -4,7 +4,7 @@ import { useState, useEffect, useMemo, useRef, useCallback } from "react";
 import DOMPurify from "dompurify";
 import { Email, ThreadGroup } from "@/lib/jmap/types";
 import { EMAIL_SANITIZE_CONFIG, collapseBlockedImageContainers, plainTextToSafeHtml, restrictDataUriResourcesOnNode, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
-import { hasMeaningfulHtmlBody } from "@/lib/signature-utils";
+import { getRenderableHtmlBody } from "@/lib/email-body-selection";
 import { collapsePlainTextQuotes, setupQuoteCollapse } from "@/lib/quote-collapse";
 import { transformInlineStyles, transformColorForDarkMode, transformBgColorForDarkMode } from "@/lib/color-transform";
 import { useThemeStore } from "@/stores/theme-store";
@@ -326,26 +326,9 @@ function EmailCard({
     if (!email) return { html: "", isHtml: false };
 
     if (email.bodyValues) {
-      let useHtmlVersion = false;
-      let htmlContent = '';
+      let htmlContent = getRenderableHtmlBody(email);
 
-      if (email.htmlBody?.[0]?.partId && email.bodyValues[email.htmlBody[0].partId]) {
-        htmlContent = email.bodyValues[email.htmlBody[0].partId].value;
-        // Prefer textBody when HTML is auto-generated minimal wrapper (no rich formatting).
-        // Server-generated HTML from text/plain emails often lacks <br> tags, collapsing newlines.
-        // Per RFC 8621, an HTML-only email exposes the same partId in both htmlBody and textBody -
-        // in that case there is no real plain-text alternative, so always render the HTML.
-        const textPartId = email.textBody?.[0]?.partId;
-        const htmlPartId = email.htmlBody[0].partId;
-        const hasDistinctTextBody = !!textPartId && textPartId !== htmlPartId && !!email.bodyValues[textPartId];
-        if (hasDistinctTextBody && htmlContent) {
-          useHtmlVersion = hasMeaningfulHtmlBody(htmlContent);
-        } else {
-          useHtmlVersion = !!htmlContent;
-        }
-      }
-
-      if (useHtmlVersion && htmlContent) {
+      if (htmlContent) {
         // Replace cid: references with authenticated blob URLs (fetched via useEffect)
         // This prevents browser auth dialogs that occur when loading raw JMAP download URLs
         if (email.attachments) {
diff --git a/lib/__tests__/email-body-selection.test.ts b/lib/__tests__/email-body-selection.test.ts
new file mode 100644
index 000000000..70210bbfc
--- /dev/null
+++ b/lib/__tests__/email-body-selection.test.ts
@@ -0,0 +1,107 @@
+import { describe, it, expect } from 'vitest';
+import { getRenderableHtmlBody } from '@/lib/email-body-selection';
+
+type PartialEmail = Parameters<typeof getRenderableHtmlBody>[0];
+
+function makeEmail(parts: {
+  html?: { partId: string; type: string };
+  text?: { partId: string; type: string };
+  values: Record<string, string>;
+}): PartialEmail {
+  const part = (p: { partId: string; type: string }) => ({
+    partId: p.partId,
+    blobId: `blob-${p.partId}`,
+    size: 1,
+    type: p.type,
+  });
+  return {
+    htmlBody: parts.html ? [part(parts.html)] : [],
+    textBody: parts.text ? [part(parts.text)] : [],
+    bodyValues: Object.fromEntries(
+      Object.entries(parts.values).map(([k, v]) => [k, { value: v, isEncodingProblem: false, isTruncated: false }])
+    ),
+  } as PartialEmail;
+}
+
+describe('getRenderableHtmlBody', () => {
+  it('renders as plain text when the shared part is text/plain (#489)', () => {
+    // Plain-text-only mail: RFC 8621 puts the same part in htmlBody and textBody.
+    const email = makeEmail({
+      html: { partId: '1', type: 'text/plain' },
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'line one\nline two\nline three' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBeNull();
+  });
+
+  it('matches the media type case-insensitively', () => {
+    const email = makeEmail({
+      html: { partId: '1', type: 'TEXT/HTML' },
+      text: { partId: '1', type: 'TEXT/HTML' },
+      values: { '1': '<p>hello</p>' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBe('<p>hello</p>');
+  });
+
+  it('renders HTML-only mail as HTML even though the part is shared', () => {
+    const email = makeEmail({
+      html: { partId: '1', type: 'text/html' },
+      text: { partId: '1', type: 'text/html' },
+      values: { '1': '<div><b>rich</b> content</div>' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBe('<div><b>rich</b> content</div>');
+  });
+
+  it('prefers the distinct text part when the HTML alternative is a bare wrapper', () => {
+    const email = makeEmail({
+      html: { partId: '2', type: 'text/html' },
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'line one\nline two', '2': '<html><body>line one line two</body></html>' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBeNull();
+  });
+
+  it('keeps a distinct HTML alternative that carries real formatting', () => {
+    const html = '<html><body><p>line one</p><p><b>line two</b></p></body></html>';
+    const email = makeEmail({
+      html: { partId: '2', type: 'text/html' },
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'line one\nline two', '2': html },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBe(html);
+  });
+
+  it('returns null when there is no HTML body at all', () => {
+    const email = makeEmail({
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'just text' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBeNull();
+  });
+
+  it('returns null when the HTML part has no fetched body value', () => {
+    const email = makeEmail({
+      html: { partId: '2', type: 'text/html' },
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'just text' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBeNull();
+  });
+
+  it('returns null for an empty HTML body value', () => {
+    const email = makeEmail({
+      html: { partId: '2', type: 'text/html' },
+      text: { partId: '1', type: 'text/plain' },
+      values: { '1': 'just text', '2': '' },
+    });
+
+    expect(getRenderableHtmlBody(email)).toBeNull();
+  });
+});
diff --git a/lib/email-body-selection.ts b/lib/email-body-selection.ts
new file mode 100644
index 000000000..02f5515ea
--- /dev/null
+++ b/lib/email-body-selection.ts
@@ -0,0 +1,35 @@
+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;
+}