The cold cost is dominated by the one-shot mount of the whole visible
window at load/switch time. 200 _ChatItemView widgets each parse Markdown,
build Content, compute height and run layout in one pass. 60 cuts that
peak ~3.3x with no loss for streaming (signature cache + throttle + render
cache keep per-token cost O(1) once mounted). Model cap (max_visible_items
* 3) follows automatically: 600 -> 180. Older items collapse into the
existing "... N earlier messages not shown" hint.