🌸 Cherry-pick request for #25332 into #25222 (Approved)

[jridgewell]

Cherry-pick request

Issue	PR	Production?	RC?	Release issue
#25332	#25337	Yes	No	#25222
#25332	#25337	No	Yes	#25319

Why does this issue meet the cherry-pick criteria?

Custom Elements inside Shadow Roots are not upgraded.

Mini-postmortem

Summary

Custom Elements inside a Shadow Root's tree (not in the document's light DOM) failed to upgrade in a very specific series:

1. Any custom is registered (eg, <amp-img>).
2. A shadow root is created
3. An unregistered custom element is appended to the shadow root (eg, <amp-state>)
4. That new custom element is registered

Users affected	Impact
PWA pages using amp-shadow	Previously unregistered elements failed to upgrade

Root Causes

1. When creating a new shadow root, the Custom Element polyfill adds the shadow root to a list of tracked "root" trees
2. When the first custom element (any custom element) is registered, we cleared the list of root trees
3. When registering any more custom elements, we used a querySelectorAll on the document to search for any already-existing elements to upgrade

Because querySelectorAll does not return results inside a shadow tree, we failed to find already-existing CEs inside them.

Action Items

Action Item	Type	Owner	PR #
A test suite for custom elements	Prevent	@jridgewell	#25370

Lessons Learned

Things that went well

• The issue was reported within 1 day of release

Things that went wrong

• Releasing the patched version took an additional day due to infrastructure issues

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/cc @ampproject/wg-approvers @ampproject/cherry-pick-approvers

[cramforce]

Approved

[jridgewell]

Postmortem posted.