Editor: Fix blocked Post Publish Panel buttons on mobile

[ivan-ottinger]

What?

Fixes #62738.

There's a small regression introduced by #62237 where on mobile viewport, the Publish and Cancel buttons are not visible.

Before	After

Why?

Without the fix, the Publish and Cancel buttons are not visible.

How?

By setting .editor-post-publish-panel's top property to $admin-bar-height-big - as it used to be

Testing Instructions

1. Switch to mobile view.
2. Start creating a new post.
3. Add some content and click the Publish button.
4. The Publish and Cancel buttons should be visible.

Testing Instructions for Keyboard

N/A

Screenshots or screencast

Shared above.

[github-actions]

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[github-actions]

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[youknowriad]

When testing this, I noticed another issue.

The Document Bar is visible on mobile and If I'm not wrong, it's supposed to be removed from mobile viewports and I'm not sure what PR restored it.

Anyway it's not a blocker for this PR.

[youknowriad]

Side thought: we definitely lack e2e tests on mobile, we should probably consider writing some of these to avoid similar regressions in the future.

[ellatrix]

I just cherry-picked this PR to the wp/6.6-rc-1 branch to get it included in the next release: 49cfe51

[ivan-ottinger]

included in the next release: 49cfe51

Thank you, Ella.