🤖 fix: wait for post-highlighting scroll RAF in CodeBlocks story

The CodeBlocks story was flaky because after Shiki highlighting
completes, code block heights change which triggers the ResizeObserver
in useAutoScroll to schedule a coalesced RAF scroll. Chromatic could
capture the snapshot before this RAF completed.

Add a double-RAF wait after verifying highlighting is done, matching
the pattern used in useAutoScroll.performAutoScroll() and the
expandAllBashTools helper.

---
_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking: `high`_

Author: ammar-agent

docs/AGENTS.md

@@ -90,6 +90,7 @@ Avoid mock-heavy tests that verify implementation details rather than behavior.
 - **Only** add full-app stories (`App.*.stories.tsx`). Do not add isolated component stories, even for small UI changes (they are not used/accepted in this repo).
 - Use play functions with `@storybook/test` utilities (`within`, `userEvent`, `waitFor`) to interact with the UI and set up the desired visual state. Do not add props to production components solely for storybook convenience.
 - Keep story data deterministic: avoid `Math.random()`, `Date.now()`, or other non-deterministic values in story setup. Pass explicit values when ordering or timing matters for visual stability.
+- **Scroll stabilization:** After async operations that change element sizes (Shiki highlighting, Mermaid rendering, tool expansion), wait for `useAutoScroll`'s ResizeObserver RAF to complete. Use double-RAF: `await new Promise(r => requestAnimationFrame(() => requestAnimationFrame(r)))`.
 
 ### TDD Expectations
 

src/browser/stories/App.markdown.stories.tsx

@@ -262,6 +262,10 @@ export const CodeBlocks: AppStory = {
       { timeout: 5000 }
     );
 
+    // Highlighting changes code block height, triggering ResizeObserver → coalesced RAF scroll.
+    // Wait 2 RAFs: one for the coalesced scroll to fire, one for layout to settle.
+    await new Promise((r) => requestAnimationFrame(() => requestAnimationFrame(r)));
+
     const url = "https://github.com/coder/mux/pull/new/chat-autocomplete-b24r";
     const container = await waitFor(
       () => {