Setting a class attribute on model elements breaks Widget handling

[rshipp]

📝

If you set a class attribute on a model element, then try to use toWidget or toWidgetEditable in the editingDowncast converter, the ck-widget class will be removed before the element is rendered in the editing view.

It took me a long time to figure this out, so I'd like to make sure it gets documented somewhere (assuming this is not a bug).

Example:

    // This breaks!
    conversion.for( 'upcast' ).elementToElement( {
        view: {
            name: 'div',
            classes: ['my-element']
        },
        model: ( viewElement, modelWriter ) => {
            return modelWriter.createElement( 'myElement', {
                'class': viewElement.getAttribute('class') || 'my-element',
            });
        }
    } );
    conversion.for( 'editingDowncast' ).elementToElement( {
        model: 'myElement',
        view: ( modelElement, viewWriter ) => {
            const moduleBlock = viewWriter.createContainerElement( 'div', {
                'class': modelElement.getAttribute('class') || 'my-element',
            } );

            return toWidget( myElement, viewWriter, { label: 'my-element widget' } );
        }
    } );

    // This works.
    conversion.for( 'upcast' ).elementToElement( {
        view: {
            name: 'div',
            classes: ['my-element']
        },
        model: ( viewElement, modelWriter ) => {
            return modelWriter.createElement( 'myElement', {
                'preserveClasses': viewElement.getAttribute('class') || 'my-element',
            });
        }
    } );
    conversion.for( 'editingDowncast' ).elementToElement( {
        model: 'myElement',
        view: ( modelElement, viewWriter ) => {
            const moduleBlock = viewWriter.createContainerElement( 'div', {
                'class': modelElement.getAttribute('preserveClasses') || 'my-element',
            } );

            return toWidget( myElement, viewWriter, { label: 'my-element widget' } );
        }
    } );

I'm happy to do a documentation update PR, but I'm not sure where is a good place to put this information. Any ideas?

[Mgsy]

cc @Reinmar

[Reinmar]

I think we stumbled upon this or a similar problem already. I don't remember, though, what was causing this problem. I found two similar issues: #4517 and #3795 for the style attribute. However, AFAIR the problem was with the view, not the model. I'm curious if in the scenario that @rshipp reported the problem lays indeed in the model or somewhere else. 

But one thing's for sure – it's definitely a bug. IMO, such things should work as one expects.

[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may still be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

We've closed your issue due to inactivity over the last year. We understand that the issue may still be relevant. If so, feel free to open a new one (and link this issue to it).