Fix: Post Editor: Canvas animates from the left when switching mobile/tablet preview to desktop

[hbhalodia]

What?

Closes #81417

Why?

• PR aims to fix the animation issue, when viewport is changed from tablet --> desktop and mobile -->desktop. It animates from the left edge, while all other animation are centrally expands.

How?

• This PR adds the little CSS fix and provide, flex-direction: column. The parent container was meant to handle centering with align-items: center, and its comment even said (flex-direction is column) — but flex-direction was never actually set. It defaults to row, where align-items: center centers vertically, not horizontally. So horizontal centering quietly fell back to that conditional margin.

Testing Instructions

Testing Instructions for Keyboard

1. Open any post or page in the Post Editor.
2. Open the viewport selector in the top toolbar.
3. Select Mobile or Tablet.
4. Switch back to Desktop.
5. Observe the editor canvas transition. This should be same as how tablet to mobile or mobile to tablet is.

Screenshots or screencast

Before

Screen.Recording.2026-08-12.at.2.55.25.PM.mov

After

Screen.Recording.2026-08-12.at.2.54.55.PM.mov

Use of AI Tools

• Claude Code, Opus 5.
• Used for initial drafting and edge case check for the animation. Implementation was reviewed and test by me.

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Detailed AI Summary

See the comment here in the issue.

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Co-authored-by: hbhalodia <hbhalodia@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: adithya-naik <adithyanaik@git.wordpress.org>
Co-authored-by: sergiu-radu <sergiuradu@git.wordpress.org>

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[t-hamano]

Thanks for the PR. I haven't tested this yet, but it looks good, so let's consider it for a minor release.