Site Editor: Unexpected "undo" behavior in StrictMode

[Mamaduka]

I noticed unexpected behavior when undoing changes in the Site Editor and React StrictMode is enabled.

After undoing the recently made changes, there are extra history records, and the template part content is gone after the changes are entirely undone. The issue affects unmodified template parts with the Navigation block.

Note

The bug doesn't affect WP 6.6 or production websites running the Gutenberg plugin.

Testing Instructions

1. Enabled SCRIPT_DEBUG so React code runs in a StrictMode.
2. Using TT4.
3. Ensure that the Header template part and template used for testing have no changes. They're loaded from a file.
4. Open a template like "Page: 404".
5. Make changes to the content or template part.
6. Undo the changes.
7. Notice that there are more "undo" steps than actual changes.
8. Fully undoing clears out the header template part.

Screencast

CleanShot.2024-07-05.at.10.18.37.mp4

CleanShot.2024-07-05.at.10.20.11.mp4

[Mamaduka]

I noticed similar behavior with the File block in the post editor, which updates attributes on the mount.

1. Enabled SCRIPT_DEBUG so React code runs in a StrictMode.
2. Insert some content.
3. Insert a file block and select file.
4. Disable the "Show download button".
5. Reload the page.
6. Notice there's an "Undo" action active.
7. Undo the changes, and the whole content disappears.

Screencast

CleanShot.2024-09-05.at.08.54.46.mp4