diff --git a/src/course-home/outline-tab/LmsHtmlFragment.jsx b/src/course-home/outline-tab/LmsHtmlFragment.jsx
index 191663554c..3c061d0fad 100644
--- a/src/course-home/outline-tab/LmsHtmlFragment.jsx
+++ b/src/course-home/outline-tab/LmsHtmlFragment.jsx
@@ -29,7 +29,7 @@ const LmsHtmlFragment = ({
   const iframe = useRef(null);
   function resetIframeHeight() {
     if (iframe?.current?.contentWindow?.document?.body) {
-      iframe.current.height = iframe.current.contentWindow.document.body.scrollHeight;
+      iframe.current.height = iframe.current.contentWindow.document.body.parentNode.scrollHeight;
     }
   }
 
diff --git a/src/course-home/outline-tab/OutlineTab.test.jsx b/src/course-home/outline-tab/OutlineTab.test.jsx
index c6fc547a0c..8d8cf453d8 100644
--- a/src/course-home/outline-tab/OutlineTab.test.jsx
+++ b/src/course-home/outline-tab/OutlineTab.test.jsx
@@ -257,6 +257,21 @@ describe('Outline Tab', () => {
       });
     });
 
+    it('ignores comments and misformatted HTML', async () => {
+      setTabData({
+        welcome_message_html: '<p class="additional-spaces-in-tag"   >'
+      + '<!-- Even if the welcome_message_html length is above the limit because of comments, we hope it will not be shortened. -->'
+      + '<!-- Even if the welcome_message_html length is above the limit because of comments, we hope it will not be shortened. -->'
+      + 'Test welcome message that happens to be longer than one hundred words because of comments but displayed content is less.'
+      + 'It should not be shortened.'
+      + '<!-- Even if the welcome_message_html length is above the limit because of comments, we hope it will not be shortened. -->'
+      + '<!-- Even if the welcome_message_html length is above the limit because of comments, we hope it will not be shortened. -->'
+      + '</p>',
+      });
+      await fetchAndRender();
+      expect(screen.getByRole('button', { name: 'Show More' })).not.toBeInTheDocument();
+    });
+
     it('does not display if no update available', async () => {
       setTabData({ welcome_message_html: null });
       await fetchAndRender();
diff --git a/src/course-home/outline-tab/widgets/WelcomeMessage.jsx b/src/course-home/outline-tab/widgets/WelcomeMessage.jsx
index ed64b1b547..f7839c5f25 100644
--- a/src/course-home/outline-tab/widgets/WelcomeMessage.jsx
+++ b/src/course-home/outline-tab/widgets/WelcomeMessage.jsx
@@ -1,4 +1,4 @@
-import React, { useState } from 'react';
+import React, { useState, useMemo } from 'react';
 import PropTypes from 'prop-types';
 
 import { injectIntl, intlShape } from '@edx/frontend-platform/i18n';
@@ -18,8 +18,22 @@ const WelcomeMessage = ({ courseId, intl }) => {
 
   const [display, setDisplay] = useState(true);
 
-  const shortWelcomeMessageHtml = truncate(welcomeMessageHtml, 100, { byWords: true, keepWhitespaces: true });
-  const messageCanBeShortened = shortWelcomeMessageHtml.length < welcomeMessageHtml.length;
+  // welcomeMessageHtml can contain comments or malformatted HTML which can impact the length that determines
+  // messageCanBeShortened. We clean it by calling truncate with a length of welcomeMessageHtml.length which
+  // will not result in a truncation but a formatting into 'truncate-html' canonical format.
+  const cleanedWelcomeMessageHtml = useMemo(
+    () => truncate(welcomeMessageHtml, welcomeMessageHtml.length, { keepWhitespaces: true }),
+    [welcomeMessageHtml],
+  );
+  const shortWelcomeMessageHtml = useMemo(
+    () => truncate(cleanedWelcomeMessageHtml, 100, { byWords: true, keepWhitespaces: true }),
+    [cleanedWelcomeMessageHtml],
+  );
+  const messageCanBeShortened = useMemo(
+    () => (shortWelcomeMessageHtml.length < cleanedWelcomeMessageHtml.length),
+    [cleanedWelcomeMessageHtml, shortWelcomeMessageHtml],
+  );
+
   const [showShortMessage, setShowShortMessage] = useState(messageCanBeShortened);
   const dispatch = useDispatch();
 
@@ -63,7 +77,7 @@ const WelcomeMessage = ({ courseId, intl }) => {
             className="inline-link"
             data-testid="long-welcome-message-iframe"
             key="full-html"
-            html={welcomeMessageHtml}
+            html={cleanedWelcomeMessageHtml}
             title={intl.formatMessage(messages.welcomeMessage)}
           />
         )}