Fix/iframed previews

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A few fixes for bugs that were introduced by WordPress 5.9.

• f5e56f6 closes Glitchy scrollbar on layout preview #684
• e2208a5 ensures the default button positioning applied to all other default layouts
• 7d13909 closes Post Inserter displaying controls on layout preview #683
• 4a0818c fixes nested iframe styles. The new iframed editor does not apply common styles when nested (iframed layout preview -> iframed posts preview). The change proposed in this commit ensure the iframe border style:

Before	After

Note that these changes will not work on Firefox due to WordPress/gutenberg#37961

How to test the changes in this Pull Request:

The bugs listed above should be fixed (in Chrome and Safari) and the layout and Post Inserter previews should continue to work as expected in 5.8.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Works as described! It also does partially fix the display in FF for me, at least by removing the border from the nested iframe.

[dkoo]

Also tested in WP 5.8 and didn't see any regressions there.

[miguelpeixe]

Thank you for reviewing, @dkoo!

[matticbot]

🎉 This PR is included in version 1.39.0-alpha.2 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.39.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀