## Rationale
`LazyMarkdownRenderer` re-parses and re-renders the **entire accumulated
message** through react-markdown on every throttled stream flush (10/s).
Render cost grows linearly with message length while streaming, so long
answers degrade progressively — this is the dominant jank vector in the
chat (verified in the perf audit: no memoization anywhere in the
message-render path).
## Why this is the root cause, not a symptom patch
The waste is structural: 99% of a streaming message is settled text that
cannot change, yet it re-renders because the whole string is one
react-markdown call. Splitting at real markdown block boundaries via
`marked.lexer` (already a dependency, used in the advanced text editor)
and memoizing per block means settled blocks keep their rendered
subtree; only the growing tail block re-parses per flush — cost becomes
O(tail) instead of O(message). Index keys are stable because streaming
is append-only. This is the standard memoized-markdown pattern from the
AI SDK ecosystem.
Deliberately **not** included: list virtualization for very long
threads. The audit's verdict was memoize first, virtualize only if
profiling still shows mount cost matters — virtualization changes scroll
behavior and deserves its own evaluation.
One known tradeoff: markdown reference-style links whose definition
lives in a *different* block won't resolve across blocks. Model output
uses inline links; the tradeoff is shared by every implementation of
this pattern.
## User impact
Long streaming answers stop stuttering — keystroke-to-paint stays flat
instead of degrading as the answer grows. Most noticeable on tool-heavy
turns that produce big final summaries.
## Test plan
- [ ] CI green (existing markdown rendering covered by storybook visual
tests)
- [ ] Manual: stream a long answer with code fences and tables —
identical rendering, no per-flush jank in the profiler
https://claude.ai/code/session_01Lyi6zTema2FMVVh8MD6c38
---
_Generated by [Claude
Code](https://claude.ai/code/session_01Lyi6zTema2FMVVh8MD6c38)_
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Félix Malfait
ab4e979352
