Cmd+A doesn't work when a document starts with checkboxes

What’s broken?

When a document starts with one or more checkbox blocks, Cmd+A followed by Backspace does not correctly clear the document content.

The issue appears to be specific to documents where a checkbox is the first block. Instead of selecting and deleting the entire document content as expected, the editor gets into a state where the content cannot be fully erased using Cmd+A + Backspace.

A screen recording demonstrating the issue is available here:

https://jam.dev/c/ac0cac24-b097-406e-8e72-f9dd43948e70

What did you expect to happen?

Pressing Cmd+A and then Backspace should select and delete all content in the document, including checkbox blocks, leaving an empty document with the default empty block.

Steps to reproduce

1. Create a new BlockNote document.
2. Add a checkbox block as the first block in the document.
3. Add some additional content below it, such as text or more checkbox blocks.
4. Focus the editor.
5. Press Cmd+A to select all document content.
6. Press Backspace.
7. Observe that the document content cannot be fully deleted.

The issue can be seen in this recording:

https://jam.dev/c/ac0cac24-b097-406e-8e72-f9dd43948e70

BlockNote version

No response

Environment

No response

Additional context

No response

Contribution

• I'd be interested in contributing a fix for this issue

Sponsor

• I'm a sponsor and would appreciate if you could look into this sooner than later 💖

Start by reproducing the issue in the BlockNote editor with a checkbox as the first block, then trace the Cmd+A and Backspace selection/deletion behavior. Add regression coverage for this document shape and verify that deleting all content leaves only the default empty block.