Visual revisions: Add a code diff view inside the editor

[priethor]

What

Closes #77841, part of #79120.

Adds a Code editor option to the block editor revisions screen, showing the selected revision's raw markup and highlighting the lines that were added or removed. The Show changes toggle on the top left switches between the diff and the plain markup.

Why

A visual diff can miss changes to block attributes when the rendered post still looks the same. The code view makes those changes visible in the new revisions screen instead of sending users back to the classic one.

Testing Instructions

1. Create a post and save two revisions with different paragraph content.
2. Open Revisions from the Post sidebar.
3. Open Options and choose Code editor.
4. Check that the old and new markup are highlighted, then move the slider and confirm the diff updates.
5. Turn off Show changes and confirm that only the selected revision's markup is shown.
6. Exit revisions while the Code editor is selected and confirm that the post editor remains in code mode.
7. Switch back to the Visual editor and confirm that revisions return to the visual view.
8. With network throttling enabled, reopen Revisions and open Options before the slider finishes loading. Confirm that the focus does not jump to the slider.

Screen recording

Grabacion.de.pantalla.2026-07-15.a.las.15.44.12.mov

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I used Codex to implement these changes. I guided the work, tested it, and reviewed the results.

[github-actions]

Size Change: +2.4 kB (+0.03%)

Total Size: 7.76 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	500 kB	+1.04 kB (+0.21%)
build/styles/editor/style-rtl.css	31.4 kB	+352 B (+1.13%)
build/styles/editor/style-rtl.min.css	26.7 kB	+329 B (+1.25%)
build/styles/editor/style.css	31.4 kB	+349 B (+1.12%)
build/styles/editor/style.min.css	26.7 kB	+330 B (+1.25%)

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[github-actions]

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Co-authored-by: priethor <priethor@git.wordpress.org>
Co-authored-by: jorgefilipecosta <jorgefilipecosta@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
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[github-actions]

Flaky tests detected in aa7aefd.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/29510630171
📝 Reported issues:

• [Flaky Test] Should navigate inner blocks with arrow keys #39607 in /test/e2e/specs/editor/various/writing-flow.spec.js
• [Flaky Test] should cut and paste individual blocks with collapsed selection #80248 in /test/e2e/specs/editor/various/copy-cut-paste.spec.js

[jorgefilipecosta]

If the request for the next revisions page fails, the core-data resolver swallows the error, getPreviousRevision keeps returning null, and this condition stays true forever, so the code view shows the spinner indefinitely (turning off "Show changes" is the only way out). I think instead of inferring the loading state from null we should check the resolution status, e.g. hasFinishedResolution( 'getRevisions', … ) on core-data, and fall back to showing the revision without the diff when the previous revision cannot be loaded.

[jorgefilipecosta]

didInteractWhileLoadingRefis set on any keydown and never reset, so a key press that does not move focus (e.g. Escape while the spinner is showing) permanently skips the slider focus and keyboard focus is left on the body. SincefocusHasNotMoved` already covers the "user moved focus" case, do we need the interaction flag at all? Could we simplify the code and remove it?

[jorgefilipecosta]

We are duplicating the ### Enhancements section, we should add the log on the existing one.

[jorgefilipecosta]

Left some comments but things look good and I think it could be merged after the comments are addressed/answred.

[ellatrix]

What do we use for the classic revisions screen? Are we using the same thing?