feat: add an opt-in MFE theme to the Text (HTML) XBlock [WIP]

[rpenido]

Description

Adds an opt-in include_theme setting to the Text (HTML) XBlock. When it is on, the block renders its author-supplied HTML inside a shadow root and applies the deployment's Paragon theme to it, so page styles cannot reach the content and the content cannot leak styles back out into the page.

Blocks created before this change, and blocks where the author leaves the setting off, render exactly as they did: the legacy html.css is loaded for them and skipped only for themed blocks.

Testing instructions

Setup

To check the feature, the deployment has to publish Paragon theme URLs the block can read, including one that cannot be missed. A single Tutor plugin does all of it and carries its own check stylesheet, so there is nothing else to create or mount. It also adds the Studio origin to the LMS CORS whitelist: Studio renders the block in an iframe on its own origin, from where the config API is not reachable unless that origin is allowed. Save this as xblock_theme_check.py in your TUTOR_PLUGINS_ROOT:

"""Publish the Paragon theme URLs the Text block's include_theme reads.
"""
import urllib.parse
from tutor import hooks

# The CSS "file", as a URL
_CHECK_URL = "data:text/css;charset=utf-8," + urllib.parse.quote(
    "h1, h2 { color: magenta !important; }"
)

_LMS_PATCH = """
# Studio renders the block in an iframe on the Studio origin, from where the
# request to the LMS config API is cross-origin, and that API is not CORS-enabled
# by default.
_studio_origin = MFE_CONFIG.get("STUDIO_BASE_URL")
if _studio_origin and _studio_origin not in CORS_ORIGIN_WHITELIST:
    CORS_ORIGIN_WHITELIST.append(_studio_origin)

_paragon_urls = MFE_CONFIG.setdefault("PARAGON_THEME_URLS", {})
_paragon_urls.setdefault("core", {"url": "https://cdn.jsdelivr.net/npm/@openedx/paragon@23/dist/core.min.css"})
_paragon_urls.setdefault("variants", {}).setdefault("light", {}).setdefault("urls", {})["brandOverride"] = __URL__
"""

_LMS_PATCH = _LMS_PATCH.replace("__URL__", repr(_CHECK_URL))

hooks.Filters.ENV_PATCHES.add_item(("openedx-lms-development-settings", _LMS_PATCH))

Enable it and rebuild:

tutor plugins enable xblock_theme_check
tutor config save
tutor dev launch -I

You will also need to have openedx/openedx-platform#39178 and openedx/frontend-app-authoring#3271 bind-mounted in your stack.

1. In Studio, open a course and add a Text (HTML) block. The Use MFE Theme toggle starts off - the setting is opt-in, so check it to turn it on for this block.
2. Type some content and save. Open the block again: the toggle must come back on, not reset.
3. With the toggle on, the editing area itself picks up the theme - a heading or link colour declared in the brand override must change inside the editor, not only in the learner view.
4. With the toggle on and the content untouched, turn Use MFE Theme off and leave it off. The block must register as unsaved even though the content never changed: Cancel must raise the "Are you sure you want to exit the editor?" confirmation, and reloading the page must show the browser's leave-page prompt. Turning it back on must then clear the unsaved state - a round trip that ends where it started is not a change, so only the first half of that is expected to be dirty.
5. Leave the toggle on and save.
6. Open the block in the learner view. The content must render inside a shadow root. Verify in the console that document.querySelector('.xblock_html').getRootNode().constructor.name returns "ShadowRoot". Note that document.querySelector('.xblock-root') returns null by design - that container is created inside the shadow root and document.querySelector does not cross the boundary; reach it with document.querySelector('.xblock_html').shadowRoot.querySelector('.xblock-root').
7. The theme must apply. A heading colour declared in the brand override stylesheet should now be visible.
8. Page CSS must not reach in. Set a distinctive body { font-family: ... } and confirm the block does not inherit it. The author's own CSS stays inside the block for the same reason. Expect one deliberate exception: the theme stylesheets are appended to document.head as well as to the shadow root, because Paragon declares its custom properties on :root and :root does not cross a shadow boundary - so seeing the theme on the rest of the page is intended, not a leak.
9. Regression check. Open an unrelated pre-existing HTML block in the same course, with the toggle off. It must render with its usual styling and no shadow root.
10. Round-trip. Export the course and inspect the block's OLX. With the setting on, the <html> element carries include_theme="true"; with it off, the attribute is absent - the serializer only writes the field when it is set, so the default never appears in OLX. This depends on fix: serialize the Text (HTML) XBlock's include_theme setting [WIP] openedx-platform#39178 - the serializer uses a hardcoded allowlist and silently drops the field without it.
11. Library authoring. Add a Text block to a library and repeat steps 1-5. The toggle, the unsaved-change warning and the save behave the same in a library as in a course.

Other information

• Depends on: fix: serialize the Text (HTML) XBlock's include_theme setting [WIP] openedx-platform#39178
• Used by: feat: add include_theme toggle to the Text XBlock [WIP] frontend-app-authoring#3271

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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Private ref: FAL-4394

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