Visual Revisions: Fix AutoSave view diff highlights unchanged blocks incorrectly - #83342

What?

Closes #80455

Fixes diff comparison when viewing an autosave revision so that unchanged content is not falsely highlighted in green as newly added.

Why?

When we observe a yellow notice: "There is an autosave of this post that is more recent than the version below."
Next to it is a button that says "View the autosave"

To render the revision diff screen that shows up upon clicking, the editor needs two pieces of data:

• A: The Autosave's content (what was typed in the autosave).
• B: The Previous content (what was saved in the post beforehand) so it can compare them.

There were two issues present:

1. The notice handler was using setCurrentRevisionId instead of openRevision, which only set state without explicitly dispatching the action to fetch the revision entity from the endpoint.
2. getPreviousRevision() resolved to null when no older revision existed in the revisions collection. This lead to a missing baseline for diff comparisons.

As a result, diffRevisionContent() and diffRevisionHTML() diffed the autosave against empty strings (""), causing the entire post (all original baseline paragraphs and the post title) to highlight in green as new additions.

How?

The issue comes from not having any previous revision in place to compare against, so in such a scenario we will compare it to the current state of the post. Thus ensuring the latest revision's changes are shown as new instead of the entire post.

1. use-autosave-notice.js: Updated the notice action click handler to dispatch openRevision( autosaveId ) so the autosave entity is properly fetched and populated into the store.

2. private-selectors.js: When the current revision is an autosave and has no older revision in the list, we will fall back to comparing against the current post content in getPreviousRevision() via getCurrentPost( state ). Normalized title, content, and excerpt to { raw: ... } to match the revision schema expected by the diff renderers.

3. Added unit tests for the selector fallback and updated notice hook tests. Updated an e2e test which would check review diff against no prior revisions.

Testing Instructions

1. Create a new draft post with some initial content:
   • Title: Sample Title
   • Content: One paragraph block with Hello world text.
2. Do not publish nor click "Save draft".
3. Add a second paragraph: Unsaved new paragraph.
4. Trigger an autosave in the browser console:

   wp.data.dispatch( 'core/editor' ).autosave();

5. Refresh the page, it will now show the "View the autosave" button after a few seconds of wait.
6. Click on "View the autosave", observe that only 'Unsaved new paragraph' is highlighted in green.

Testing Instructions for Keyboard

N/A

Screenshots or screencast

Before	After
For the default Hello World post
Followed via the testing instructions - new draft post

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Gemini 3.7 Flash (Low, Medium)
Used for: Help in identifying the error and locating the relevant code lines to fix, approach of fixing. Unit tests were written via AI. Final implementation was reviewed and tested by me.