The last status-dependent read at the message root, and the most expensive
one: the completedText selector flipped between '' while running and a full
messageContentText(content) join once settled, so every running <-> settled
transition re-ran the join for the whole message AND re-rendered the root. At
stream breadth N that is N joins plus N root re-renders per flip.
completedText and the previewTargets memo it feeds now live in a new
AssistantPreviewEmbeds leaf, mounted at the same position inside
[data-slot='aui_assistant-message-content']. Verified before moving that
previewTargets fed nothing else at the root -- its only consumer was its own
render block. The leaf renders the same wrapper div with the same classes, or
null when there are no targets, so the DOM is byte-identical; a component
boundary adds no node, so unlike StreamingMarker this needed no placement care
around the :first-child/:last-child rules.
The '' branch is preserved deliberately: it is the streaming-side optimization
that keeps the selector referentially stable so per-token flushes skip the
regex scan.
AssistantMessageBody now holds no status-dependent subscription at all -- what
remains is messageId, hasVisibleText, isInterim and turnDurationS, none of
which move on a pending flip.
Adds preview-embeds.test.tsx. The embed had no coverage, and the two cases are
written as a matched pair on the same selector -- present once settled, absent
while running -- so neither can pass vacuously.
Behavior-identical; invalidation scope only.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Royalaid
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