## Summary
- Changed `navigateToRoute()` to use `postMessage` +
`location.replace()` pattern instead of direct `location.href`
assignment
- This matches the approach used by back/forward navigation buttons and
provides smooth navigation without the black screen flicker and sidebar
state reset
- Also added proper `currentIframeUrlRef` and `preservedUrls` updates
for HMR remount consistency

Fixes #2428

## Test plan
- [x] Build succeeds
- [x] Unit tests pass (784 tests)
- [x] E2E tests pass for preview navigation (`npm run e2e -- --grep
"preview navigation"`)
- Manual testing: Open a multi-page app, use the route dropdown to
navigate between routes - should no longer see black screen flicker or
sidebar collapse

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes #2428: removes black screen flicker and sidebar collapse when
selecting routes from the preview dropdown. Navigation now uses
postMessage + location.replace and keeps the iframe src stable across
SPA navigation and HMR.

- **Bug Fixes**
- Use postMessage + location.replace to navigate; prevents flicker and
preserves sidebar state.
- Freeze iframe src with useMemo and same-origin check so SPA nav/HMR
don’t reset it; added e2e test to verify src remains unchanged.
- Sync navigation state: history, canGoBack/canGoForward,
currentIframeUrlRef, and per-app preservedUrls (clears on root).

<sup>Written for commit 1bdd7635ea432c564fe77fea57062df1a9d2f56d.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

---------
Co-authored-by: Will Chen <willchen90@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>