feat: adds scrollToXBlock message handler for cms iframe view

[tecoholic]

Description

This commit adds a new message handler to the XBlockContainerPage CMS view, that allows the MFE to send a signal to the IFrame and scroll to a specific XBlock.

Useful information to include:

• Which edX user roles will this change impact? "Course Author"

PoC showing an MFE plugin that makes use of this message handler to scroll the page to specific XBlocks.

Screencast_20250403_155811.webm

Supporting information

Link to other information about the change, such as Jira issues, GitHub issues, or Discourse discussions.
Be sure to check they are publicly readable, or if not, repeat the information here.

Testing instructions

By default, the Authoring MFE uses legacy Studio UI for the Course Unit page. In order to test the
PR changes, enable the MFE's course unit page.

1. Create a Course Waffle Flag in the LMS admin (/admin/waffle_utils/waffleflagcourseoverridemodel/) with:
   • Waffle Flag: contentstore.new_studio_mfe.use_new_unit_page
   • Course ID: The ID of the course you are using for testing.
   • Override Choice: Force ON
   • Enabled: Check
2. Add a .env.private in the MFE repo with the following

ENABLE_UNIT_PAGE=true

3. Now go to the Studio and open the "Unit page" of a course.
4. Use the inspector and copy a XBlock ID for testing. (preferable one that's in the bottom of the page)
5. Now switch to the JS Console and use the following snippet to post a message. The page should scroll to the XBlock

let blockId = "<your-block-id>";
let iframe = document.getElementsByName("xblock-iframe")[0];
iframe.contentWindow.postMessage({type: "scrollToXBlock", payload: { locator: blockId }}, target="http://studio.local.openedx.io:8001")

Make sure that the context it "top" pointing to the MFE in the console, and adjust the target value to your studio URL.

Deadline

None

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
  NO

• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.
  N/A

• If your database migration can't be rolled back easily.

[openedx-webhooks]

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[farhaanbukhsh]

👍

• ✅ I tested this on local tutor stack
• ✅ I read through the code
• ❌ I checked for accessibility issues
• ❌ Includes documentation

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.