Problem: Discarding a document that contains an SVG “use” element could
abort the process with a !is_in_list() verification failure in the
IntrusiveListNode destructor. That surfaced intermittently in our style-
invalidation stress tests, depending on GC sweep order.
Cause: A “use” element connected to a document registers itself in the
document’s list of “use” elements and unregisters during its removal
steps. A GC’ed “use” element is swept without running those removal
steps — so it stays linked. When it’s destroyed before its document,
its list node is still linked — and the destructor aborts.
Fix: Override finalize() to unregister the “use” element before
destruction. The collector finalizes every dying cell before destroying
any of them. So, the node is always unlinked in time — the same approach
DocumentObserver and NavigationObserver already use.
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