fix(ui): persist trace step-iframe split pane sizes

[hi-ogawa]

Description

• follow-up to feat(ui): persist trace view selection in URL #10981

This is a final missing piece to make trace view stable after reload.

Demo from https://github.com/vitest-dev/vitest/actions/runs/32683521351/job/97305462075?pr=11027

Screen.Recording.2026-08-24.at.11.45.30.mov

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.
• Please check Allow edits by maintainers to make review process faster. Note that this option is not available for repositories that are owned by Github organizations.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	045da8d
🔍 Latest deploy log	https://app.netlify.com/projects/vitest-dev/deploys/6a8817ba8cf5570008267b40
😎 Deploy Preview	https://deploy-preview-11027--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pkg-pr-new]

@vitest/browser

npm i https://pkg.pr.new/@vitest/browser@c36c4fd

@vitest/browser-playwright

npm i https://pkg.pr.new/@vitest/browser-playwright@c36c4fd

@vitest/browser-preview

npm i https://pkg.pr.new/@vitest/browser-preview@c36c4fd

@vitest/coverage-istanbul

npm i https://pkg.pr.new/@vitest/coverage-istanbul@c36c4fd

@vitest/coverage-v8

npm i https://pkg.pr.new/@vitest/coverage-v8@c36c4fd

@vitest/expect

npm i https://pkg.pr.new/@vitest/expect@c36c4fd

@vitest/mocker

npm i https://pkg.pr.new/@vitest/mocker@c36c4fd

@vitest/pretty-format

npm i https://pkg.pr.new/@vitest/pretty-format@c36c4fd

@vitest/snapshot

npm i https://pkg.pr.new/@vitest/snapshot@c36c4fd

@vitest/spy

npm i https://pkg.pr.new/@vitest/spy@c36c4fd

@vitest/ui

npm i https://pkg.pr.new/@vitest/ui@c36c4fd

@vitest/utils

npm i https://pkg.pr.new/@vitest/utils@c36c4fd

vitest

npm i https://pkg.pr.new/vitest@c36c4fd

@vitest/web-worker

npm i https://pkg.pr.new/@vitest/web-worker@c36c4fd

commit: c36c4fd

[sheremet-va]

local storage is not persistent between different runs, is it? we also have the same issue with dark mode. these settings should probably be also stored on the device somewhere

not blocking though

[sheremet-va]

Conflict! 👀