Switching to an already-open session remounted the incoming transcript and
re-tokenized every fenced code block from scratch on the main thread (N blocks
x full shiki tokenization per switch, 96-100% CPU for seconds).
- shiki-block: content-keyed LRU cache of highlighted HTML (theme scope +
language + code); remounts of unchanged blocks paint cached markup with
zero highlighter calls; misses debounced, failures degrade to plain text
- use-session-actions: warm resume keeps the session-slice array when the
reconciled content is equivalent (same guard as the cold path), so the
runtime repository and every row keep identity
- transcript-window: per-session sticky window memos; a warm re-visit with an
unchanged transcript reuses the windowed slice by reference (no re-index,
no repository rebuild); sticky cut survives switches for sessions that grew
- perf regression guards: remount must not re-tokenize (codeToHtml called
once per unique block), windowed slice reference preserved across switches,
messageComponents identity stable across session switches
Finn763
e245e40f73