What?

See #65778

Experiments with allowing list items to be added/removed and outdented/indented in content only mode

How?

Adds a contentOnlyInsertion property to useInnerBlocksProps. (maybe allowContentOnlyInsertion is a better name)

When this is present, it bypasses some of the usual logic for blocks in the canInsertBlockTypes and canRemoveBlock selectors.

Notes

• At the moment this works ok for list as it has a limited subset of inner block types, but for other blocks like quote that accept any inner block type, we might want to limit what can be inserted in contentOnly mode. Perhaps an option is an API like allowContentOnlyInsertion: [ 'core/paragraph' ]
• There might be some writing flow oddities to consider. I think I noticed a few, but I need to write them down
• When you have a list nested in another list, the 'section' (or content only locked parent) content panel should probably only show the parent-most list, at the moment it'll show the nested one too, but they look like siblings:

• Similarly List View should probably be flattened
• Is useInnerBlocksProps the right place to declare this or should it be in the block.json? With useInnerBlocksProps it can at least be made private to start with.

Testing Instructions

1. Open a template like Blog Homepage in the site editor
2. Add a List in the middle of one of the sections/patterns
3. Switch to write mode an try editing the List

Also try with contentOnly locked group, here's a code snippet that can be pasted into a code editor:

<!-- wp:group {"templateLock":"contentOnly","layout":{"type":"constrained"}} -->
<div class="wp-block-group"><!-- wp:list -->
<ul class="wp-block-list"><!-- wp:list-item -->
<li>1<!-- wp:list -->
<ul class="wp-block-list"><!-- wp:list-item -->
<li>2</li>
<!-- /wp:list-item --></ul>
<!-- /wp:list --></li>
<!-- /wp:list-item -->
<!-- wp:list-item -->
<li>3</li>
<!-- /wp:list-item -->
<!-- wp:list-item -->
<li>4</li>
<!-- /wp:list-item --></ul>
<!-- /wp:list --></div>
<!-- /wp:group -->

Screenshots or screencast