fix(ui): prevent structured Markdown reflow during post-terminal handoff #731

Problem

PR #711 replaced full-document streaming Markdown rendering with Streamdown block rendering and materially stabilized the active streaming phase. However, current main still performs a large structural reflow after the runtime reaches a terminal state, while the live answer completes its catch-up / handoff.

This is narrower than the earlier #646 field finding:

• active provider streaming is now stable;
• the remaining jump occurs after Stop has disappeared;
• structured Markdown changes from a temporary single block into its final block tree in one frame.

Current evidence

Measured on current main (d736901d) through the production Electron renderer path using the deterministic MAKA_E2E FakeBackend:

• FakeBackend emitted 9-character deltas every 45 ms.
• The lazy Markdown module was warmed before measurement.
• .maka-markdown-root geometry and structure were sampled every 8 ms.
• Table, multi-paragraph, and unstructured controls ran in isolated app sessions.

At the table jump, the Markdown root changed in one frame from:

P

to:

P / P / DIV(table) / P

The Stop control was already absent before and after that frame, locating the failure in post-terminal completion rather than provider delta delivery.

Relevant paths:

• packages/ui/src/markdown-body.tsx
• packages/ui/src/chat-view.tsx
• packages/ui/src/smooth-stream.ts

Goal

Keep already-visible Markdown blocks visually stable when a live answer completes, without changing the final settled Markdown semantics.

Scope

• GFM tables.
• Multi-paragraph Markdown.
• The post-terminal live catch-up / settled handoff.
• Production Streamdown rendering and the existing single live-turn render path.

Acceptance criteria

• A completed paragraph preceding a trailing table or later paragraph keeps the same DOM ownership and moves by no more than 1 CSS px during completion.
• A table answer does not replace a temporary whole-answer <p> with the final multi-block tree in one frame after terminal state.
• Final settled Markdown content, table semantics, link safety, highlighting, and copy behavior remain unchanged.
• Active-stream behavior from fix(ui): stabilize streaming turn order and markdown layout #711 remains bounded and does not regress.
• A deterministic DOM geometry test covers the table and multi-paragraph cases, with an unstructured-text control.
• Verification uses temporal DOM measurements; a static screenshot alone is not sufficient.

Non-goals

• Provider delta coalescing or stream-speed tuning.
• Markdown typography or visual redesign.
• Headings and fenced code blocks until they have independent current-main reproductions.
• A parallel Markdown render path or custom parser.