Fix: Delete key on empty heading transforms following paragraph into heading

[coderGtm]

What?

Closes #78767

Fix a bug where pressing Delete on an empty heading block incorrectly transforms the following paragraph block into a heading.

Why?

When a heading block is emptied and the user presses Delete, the expected behaviour is that the empty heading is removed and the paragraph below is left unchanged. Instead, the paragraph was being converted into a heading — matching the type of the block that was being deleted.

This only occurred with the Delete key (forward merge). Backspace was unaffected because pressing Backspace on an empty heading with no preceding block triggers switchToDefaultOrRemove(), which converts the heading itself to a paragraph without ever reaching the merge path.

How?

The bug was in the mergeBlocks action (packages/block-editor/src/store/actions.js). When mergeBlocks(blockA, blockB) is called, blockA's type is preserved and blockB's content is merged in. Two existing guard conditions short-circuit this behaviour:

• isUnmodifiedDefaultBlock(blockA) → removes blockA (handles an empty default/paragraph block)
• isUnmodifiedDefaultBlock(blockB) → removes blockB (handles an empty following block)

Neither condition matched for an empty heading (blockA) above a paragraph with content (blockB): a heading is not the default block type, so the first check failed, and the paragraph had content, so the second also failed. The code then fell through to the merge logic, which calls switchToBlockType(blockB, blockA.name) — transforming the paragraph into a heading before merging.

The fix adds a new guard: if blockA has a merge function but its content-role attributes are empty (isUnmodifiedBlock(blockA, 'content')), simply remove blockA and focus blockB instead of merging. A unit test covering this case is included.

Testing Instructions

1. Open a post or page in the block editor.
2. Insert a Heading block and type any text (e.g. "My Heading").
3. Insert a Paragraph block below it and type any text (e.g. "My paragraph").
4. Click into the Heading block and select all the text (Cmd/Ctrl+A), then press Delete/Backspace to clear it. The heading block should now be empty.
5. With the cursor still in the empty heading block, press the Delete key (forward delete).
6. ✅ Expected: The empty heading block is removed and the paragraph block below remains a paragraph with its content intact.
7. ❌ Previous behaviour: The paragraph was converted into a heading.

Testing Instructions for Keyboard

1. Use Tab to navigate to the Heading block and Enter to begin editing.
2. Press Cmd/Ctrl+A to select all text, then Backspace to clear the heading content.
3. Press Delete (forward delete).
4. Confirm focus moves to the paragraph below and it remains a paragraph block (check the block type indicator in the toolbar).

Screenshots or screencast

Before	After
Screen.Recording.2026-05-28.at.7.24.19.PM.mov	Screen.Recording.2026-05-28.at.7.31.40.PM.mov

Use of AI Tools

This PR was developed with the assistance of GitHub Copilot (code exploration, fix implementation, and test authoring). The change was reviewed and verified manually.

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Co-authored-by: coderGtm <gautam23@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>
Co-authored-by: Mustafabharmal <mustafabharmal@git.wordpress.org>

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[jasmussen]

Nice, thanks for the PR! Confirming it works for me, and I personally appreciate this a great deal as far as the writing experience.

Since this touches a sensitive part of the code, I will ultimately defer to developers familar with this code to approve or not. @ellatrix has worked a lot on writing flow.

[youknowriad]

Can you check the e2e test failures? It seems this could have introduced regressions on the list block no?

[ntsekouras]

We probably need to check if blockA.name !== blockB.name and also it would be better to have the code below the if ( ! blockAType.merge ) { block for two reasons:

1. We remove the blockAType.merge check
2. Preserve the if ( isUnmodifiedDefaultBlock( blockB ) ) { block flow. Placement here changes the current behavior a bit right now and we can avoid it.

[coderGtm]

Thanks @ntsekouras! That actually is a better way. I made the change and tested the behaviour and It works flawlessly.

[ntsekouras]

Failing list tests seem that something different (or more) is needed. --cc @ellatrix for any quick insights due to the heavy work on this area.

[coderGtm]

So I have fixed the logic and updated the test. There is still a keyboard testcase failing which I will try solving.