Post Editor: Canvas animates from the left when switching mobile/tablet preview to desktop #81417

Description

When switching the Post Editor viewport from Mobile → Desktop or Tablet → Desktop, the editor canvas expands from its left-aligned device-preview position before moving into its final centered desktop position.

This makes the canvas appear to slide from left to center/right during the transition.

The other viewport transitions behave as expected:

• Desktop → Tablet
• Tablet → Mobile
• Mobile → Tablet

The issue appears to affect the editor canvas transition itself, rather than the content inside the canvas.

Expected behavior

The editor canvas should remain centered while transitioning from a mobile or tablet preview to the desktop preview.

Actual behavior

When switching from Mobile or Tablet to Desktop, the canvas first grows from the left side and then moves into its centered desktop position.

Step-by-step reproduction instructions

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.

The issue occurs with both Mobile → Desktop and Tablet → Desktop transitions.

Screenshots, screen recording, code snippet

The issue is most visible when switching from Mobile or Tablet back to Desktop.

Environment info

• WordPress version: 7.1 RC2
• Gutenberg version: Not installed separately; using the editor bundled with WordPress core
• Browser: Chrome
• Operating system: macOS
• Theme: Twenty Twenty-Five version 1.5

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure