[Plain table output]: The PlainTableOutput overrides downcast converters of other features

[Reinmar]

Why

This ticket is a follow-up to #11240.

There's a bug in the converter that it affects conversion of <caption> elements in other widgets than table.

Use this below code (excerpt from ImageCaptionEditing):

// Model -> view converter for the data pipeline.
editor.conversion.for( 'dataDowncast' ).elementToElement( {
    model: 'caption',
    view: ( modelElement, { writer } ) => {
        const imageUtils = editor.plugins.get( 'ImageUtils' );
        if ( !imageUtils.isBlockImage( modelElement.parent ) ) {
            return null;
        }

        return writer.createContainerElement( 'foobar' );
    },
    converterPriority: 'high'
} );

It should override ImageCaptionEditing's converter to produce the <foobar> element instead.

It works without the PlainTableOutput plugin but fails with it (if the order in which plugins are loaded is PlainTableOutput, ImageFooBar).

[Reinmar]

Let's merge this to the release branch because it seems to be a blocker.

[dawidurbanski]

@Reinmar Pull request is ready to review: #11407

[dawidurbanski]

This was happening due to the fact that the PlainTableOutput downcast converter was unnecessary creating <figcaption> element for all model elements other than table.

Since this converter is registered with the high priority, only higher priority converters would actually be able to override this behaviour. In case of other high priority converters it was a plugin loading order issue.

The solution was to just simply do nothing inside the converter callback if we detect we are within an element other than table.