Description

Fixes #36710; possible alternative to #37934.

When focus is on a block with a placeholder (e.g. Image or Columns block) and we are in Edit mode, we should be able to Tab into the placeholder controls. This makes the placeholder controls keyboard accessible when the "Contain text cursor inside block" preference is enabled (arrow key navigation in or out of a block doesn't work with that preference). It also makes it possible to tab into a block that only contains a block inserter, such as an empty Group block.

Testing Instructions

1. In a post, add a Columns block.
2. Check that pressing Tab moves focus to the first control in the placeholder.
3. Select an option from the placeholder, e.g. "Two columns; equal split".
4. Focus should now be on the first column.
5. Check that pressing Tab moves focus to the block inserter inside the first column.
6. Add a Group block.
7. Check that pressing Tab moves focus to the block inserter inside the Group block.

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.