A multi-MB message (logged bundle, huge tool dump) froze the renderer
before any paint: Streamdown runs `preprocess` + `marked` lex over the
whole string synchronously in a useMemo, an uninterruptible long task
that no try/catch or content-visibility can help (our JS runs before the
browser ever skips layout). Tiered fix:
- Message gate: past 200KB, bypass markdown entirely and render the raw
text in `content-visibility:auto` line-chunks — synchronous work is
bounded to a string split, the browser virtualizes layout natively,
and every line stays in the DOM (selectable, find-in-page).
- Code-block budget: past 3k lines / 150KB, skip Shiki (which emits a
span per token) and render plain, chunked the same way.
- Collapse/expand: a reusable ExpandableBlock clamps code blocks and the
huge-text fallback to a 120px preview with a gradient + chevron,
expanding to 300px. The inner element is always a scroll container so
the content-visibility chunks stay lazily laid out in both states.
No content is ever dropped; the copy button (card header) always yields
the full block.
0138282f97