Fix: Retain focus and caret position after Backspace undo of a prefix block transform

[SainathPoojary]

What?

Closes #82079

Fixes lost focus and caret position when you press Backspace right after a prefix transform (like ## , - , or > ) turns a Paragraph into a Heading, List, or Quote block.

Why?

When a prefix transform runs, the block is replaced and the cursor is moved to the new block, but these two things were dispatched as separate, unbatched updates. Because of that, the undo history recorded the new block's cursor position as both the "before" and "after" state, instead of the original block's. So when Backspace triggered undo, Gutenberg tried to restore focus using a block that no longer existed after the undo, and focus was lost. The user then had to click back into the block to keep typing, which is also an accessibility problem.

How?

selectionChange() and onReplace() in input-rules.js now run inside a single registry.batch() call, so they are recorded as one atomic update. This way the undo history correctly keeps the original block's cursor position as the "before" state, and restoreSelection() can put the caret back where it was after Backspace-undo.

Testing Instructions

1. Open a post or page.
2. Click into the empty paragraph block.
3. Type ## (hash, hash, space). The block turns into a Heading.
4. Press Backspace. The block turns back into a Paragraph.
5. Without clicking anywhere, start typing. The text should be added right where the cursor was, no need to click back in.
6. Repeat with - (List) and > (Quote) to confirm the same fix applies to other prefix transforms.
7. Try the same steps with the paragraph nested inside a Group block, to confirm focus is retained there too.

Screenshots or screencast

Screen.Recording.2026-08-27.at.2.40.30.PM.mov

Use of AI Tools

Used AI assistance (GitHub Copilot) to help write PR description and validate the fix and tests. I reviewed the analysis, tested the change manually and with the added e2e tests, and I take responsibility for the code in this PR.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: SainathPoojary <sainathpoojary@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[Mamaduka]

Thanks for working on this, @SainathPoojary!

Left couple of questions regarding the new e2e tests.