Pasting Mixed Tagname HTML Breaks if 'core/heading' Block Has Been Disabled

[Kelderic]

Describe the bug
When using the default 'core/paragraph' and 'core/heading' blocks, I can copy text from another website that includes <p> tags and <h2> tags mixed within the copied content. I can then paste it into Gutenberg and the content is properly split. However, if the 'core/heading' is disabled, (say to replace it with a custom one), then pasting stops working altogether if the pasted content contains mixed text.

To reproduce
Steps to reproduce the behavior:

1. Disable the 'core/heading' block.
2. Add custom block using RichText, with <h2> as the wrapper.
3. Copy the following HTML: <p>First line</p><h2>Second line.</h2><p>Third line</p>
4. The console will log that the content has been properly converted. But nothing will happen and the paste breaks.

Expected behavior
I would expect the content to be pasted into three blocks. A paragraph, a custom heading, and a paragraph. Or, I'd at least expect a console error message.

Actual behavior
Nothing happens. Pasting just does nothing.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• Browser Firefox
• Version 71.0

Custom block RichText Element:

		        <RichText 
		            tagName="h2"
		            withoutInteractiveFormatting={ true }
		            allowedFormats={ [] }
		            className={ className }
		            value={ attributes.content }
		            onChange={ ( value ) => blockProps.setAttributes( { content: value } ) }
		            style={ { textAlign: attributes.alignment } }
		            placeholder={ 'Write heading...' }
		            keepPlaceholderOnFocus={ true }
		        />

Does there need to be a way to register a custom block as the recipient of certain HTML paste types?

[Kelderic]

The code that I am using to disable the 'core/heading' block is:

// ONLY ALLOW CERTAIN BLOCKS

add_filter( 'allowed_block_types', 'allowed_block_types' );
 
function allowed_block_types( $allowed_blocks ) {
 
	return array(
		'core/block',
		'core/image',
		'core/paragraph',
		'core/list',
		'core/quote',
		'core/video',
		'core/code',
		'core/table',
		'core/separator'
	);
 
}

[talldan]

@Kelderic I think this could be considered a bug. Ideally the editor wouldn't just ignore the pasted content even when some blocks are disabled. I think the Classic block or Custom HTML block could be considered fallbacks for content that can't be matched to a block, but the paste was still ignored when I tried with those blocks enabled.

Does there need to be a way to register a custom block as the recipient of certain HTML paste types?

Yep, that's handled using the transform system as a 'raw' transform. Here's how the Heading block handles it:

gutenberg/packages/block-library/src/heading/transforms.js

Lines 26 to 58 in 21936b5

	{
	type: 'raw',
	selector: 'h1,h2,h3,h4,h5,h6',
	schema: ( { phrasingContentSchema, isPaste } ) => {
	const schema = {
	children: phrasingContentSchema,
	attributes: isPaste ? [] : [ 'style' ],
	};
	return {
	h1: schema,
	h2: schema,
	h3: schema,
	h4: schema,
	h5: schema,
	h6: schema,
	};
	},
	transform( node ) {
	const attributes = getBlockAttributes( name, node.outerHTML );
	const { textAlign } = node.style || {};
	attributes.level = getLevelFromHeadingNodeName( node.nodeName );
	if (
	textAlign === 'left' ||
	textAlign === 'center' ||
	textAlign === 'right'
	) {
	attributes.align = textAlign;
	}
	return createBlock( name, attributes );
	},

Unfortunately there aren't a lot of docs for raw transforms, but the main bit you'd be concerned about is the transform function and creating your block with the right attributes instead of the core/heading block.

[Mamaduka]

The root cause of the issue is probably the same as for #11245 and #27835.