ui : mount chat messages lazily near the viewport
Every message row mounted its full component tree on load, so the cycle
collector, GC and layout invalidation kept walking every live object and
DOM node even for rows the user never scrolls to - which dominated the
profile of long conversations. Wrap each row in a placeholder with an
IntersectionObserver ( two viewport heights of runway ) that swaps in the
real ChatMessage when the row approaches the viewport; the row shell
keeps the content-visibility sizing, and rows stay mounted once
realized. Rows targeted by the pending-edit flow mount eagerly.
Assisted-by: pi:zai-org/GLM-5.3
