Fix parser registry lookup for declarative shadow root templates

Jayson Chen · chromium-wpt-export-bot · commit 4a13b6cb08ef · 2026-04-24T13:19:34.000-07:00
The parser optimization in 2f14d02eaed47 changed the template check in HTMLConstructionSite::CreateElement from catching all template elements to only catching fragment-parsing templates or template content documents. This missed declarative shadow root templates, where SetOverrideInsertionTarget redirects output to the shadow root in the main document (so is_parsing_fragment_ is false and document.IsTemplateDocument() is false). As a result, elements inside declarative shadow root templates were getting the wrong registry from CurrentElement()->customElementRegistry() (which resolves via the template element's tree scope — the parent shadow root) instead of getting the registry from their actual target shadow root. Fix by distinguishing three cases when CurrentNode is a template: 1. Declarative shadow root template (IsShadowRootModeTemplate): use the target shadow root's own customElementRegistry(), which correctly returns null for scoped-waiting roots and the global registry for non-scoped roots. 2. Regular <template>: null the registry unconditionally, since template content documents have no browsing context. 3. Deeper descendants inside template content documents: null the registry (existing IsTemplateDocument check). Additionally, set ScopedCustomElementRegistryUsed() on the document when attaching a declarative shadow root with shadowrootcustomelementregistry, so that the per-element registry lookup path is activated during parsing. Add a test that an element's null customElementRegistry does not mutate after removal from a null-registry declarative shadow root. Bug: 490343466 Change-Id: I7878bfe187bea2cb6eb4f63d443685147a3c235b Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7774289 Commit-Queue: Jayson Chen <jaysonchen@microsoft.com> Reviewed-by: Joey Arhar <jarhar@chromium.org> Cr-Commit-Position: refs/heads/main@{#1620385}
diff --git a/custom-elements/registries/element-mutation-null-registry-removal.html b/custom-elements/registries/element-mutation-null-registry-removal.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<title>An element's null customElementRegistry should not mutate after removal</title>
+<meta name="author" title="Jayson Chen" href="mailto:jaysonchen@microsoft.com">
+<link rel="help" href="https://wicg.github.io/webcomponents/proposals/Scoped-Custom-Element-Registries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<body>
+<div id="host">
+    <template shadowrootmode="open" shadowrootcustomelementregistry>
+        <x-el></x-el>
+    </template>
+</div>
+
+<script>
+
+test(() => {
+    const sr = host.shadowRoot;
+    const el = sr.querySelector('x-el');
+    assert_equals(el.customElementRegistry, null,
+        'customElementRegistry should be null while in null-registry shadow root');
+    el.remove();
+    assert_equals(el.customElementRegistry, null,
+        'customElementRegistry should remain null after removal from null-registry shadow root');
+}, "An element's null customElementRegistry should not mutate after removal from a declarative shadow root with shadowrootcustomelementregistry.");
+</script>
+</body>
