UI: Fix horizontal scroll bar in Canvas hidden by styling

[yoshi2no]

Closes #24226

What I did

• remove negative margin from ChildrenContainer in code/ui/blocks/src/components/Preview.tsx
  • This bug was happening when the layout wascentred, orpadded.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

1. Run a sandbox for template nextjs-default-ts
2. Set width to in sandbox/nextjs-default-ts/stories/Header.tsx (for scrolling) like this

<header style={{width: "1200px"}}></header>

3. Temporarily set the layout of sandbox/nextjs-default-ts/stories/Header.stories.ts to padded or centered.

const meta = {
  // omitted
  parameters: {
    layout: 'centered', // or 'padded'
  },
} satisfies Meta<typeof Header>;

4. Open Storybook in your browser
5. Access story http://localhost:6006/?path=/docs/example-header--docs

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[cdedreuille]

After testing it, I'm not sure this is the right solution.

[cdedreuille]

Actually, it does work. I'll accept it again :) The changes pushes the content 10px more inside the canvas which is quite a big change but it also fixes the problem of having the button "Show code" overlapping on the content. If we update the placement of this button in the future, we might be able to reduce the paddings again.

Thanks a lot for making that change @yoshi2no