Undo (Ctrl+Z) in Site Editor clears all content unexpectedly after multiple undos #73194

Description

When using the WordPress Site Editor, pressing Ctrl+Z (Undo) multiple times causes the editor content to disappear completely — instead of reverting changes step by step.

This happens both while editing page content and when editing Additional CSS in the global styles panel.

Step-by-step reproduction instructions

Case 1 — Editor Content

1. Open Site Editor → Edit any page or template.
2. Modify text or block content (e.g., edit a paragraph).
3. Press Ctrl+Z multiple times (5–6+ times).
4. Observe: all content remove, editor is blanked.

Case 2 — Additional CSS

1. Open Site Editor → Styles → Additional CSS.
2. Add a few lines of CSS.
3. Press Ctrl+Z multiple times.
4. Observe: all content remove, editor is blanked.

Screenshots, screen recording, code snippet

https://www.awesomescreenshot.com/video/46299475?key=d984c27acd2d357b283f91842e52429c
https://www.awesomescreenshot.com/video/46299586?key=004b7f96b74dbaea49976ae401273c7f

Environment info

WordPress version: 6.8.3
Browser: Chrome 142.0.7444.134 (Linux)
OS: Ubuntu 22.04.5 LTS
Active theme: Twenty Twenty-Five
No other plugins

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