slotFor() cannot handle text nodes

[Reinmar]

    editor.model.schema.register( 'myElement', {
        inheritAllFrom: '$block'
    } );

    editor.conversion.for( 'downcast' ).elementToStructure( {
        model: 'myElement',
        view: ( modelElement, { writer, slotFor } ) => {
            return writer.createContainerElement( 'div', { class: 'wrapper' }, [
                writer.createContainerElement( 'p', null, [
                    slotFor( 'children' )
                ] )
            ] );
        }
    } );

    setData( editor.model, '<myElement>foo[]</myElement>' );

I'd expect this in the view:

<$root>
    <div class=wrapper>
        <p>foo[]</p>
    </div>
</$root>

Instead, we get this:

<$root>
    <div class=wrapper>
        <p>foo</p>[]
    </div>
</$root>

NOTE: Exactly the same problem will occur when trying to use slots to handle <pre><code> in the code block feature.

[Reinmar]

Ideas:

• A: Model to view selection conversion: pass a flag that we're looking for an innermost position.
  • A[<myElement>]B[</myElement>]C
  • A[<div class=wrapper><p><ui/>]B[<ui/></p></div>]C
  • Algorithm:
    • When slotFor() is used, remember on which index of its parent it was defined
    • Map children/positions of <myElement> relatively to that memorized position (<p> + index)
  • Cons: If we need to memorize such things then why remove slot elements in the first place?
• B: Introduce support for single model element to two view elements mapping (the outermost and innermost).
  • What about multiple slots?
  • What about <p><x>XYZ</x>slotFor('children')<x>XYZ</x></p>?
    • There's no innermost element!
  • This should be forbidden: <myElement><$text a=1>X</$text><$text b=1>Y</text></myElement> 😱
    • In this case, multiple slots should be forbidden.
• C: What if slots weren't removed on conversion?
  • Will it cause the same problems but on the view<->DOM step instead of on the model<->view?
  • What kind of element this would need to be?
  • It makes sense to research this.
• D: Make slots handle only elements, not text nodes.
  • Reasonable for now.
  • Throw when a text node was encountered by slotFor().

[Reinmar]

D: Make slots handle only elements, not text nodes.

• Reasonable for now.
• Throw when a text node was encountered by slotFor().

Extracted to: #11163.

[Reinmar]

I've recently stumbled upon #11044 where the person wants to turn <p>s into <p><span>...</span></p>. The purpose, as far as I know., is the ability to use overflow-wrap.

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