Attributes are set to all the element's children during upcast

[scofalik]

Originally reported on SO: https://stackoverflow.com/questions/50992491/ckeditor5-prevent-nested-same-type-elements-from-automatically-inheriting-paren

The issue is that during upcast we use data.modelRange which holds the range over the just-inserted elements so the converter knows where to operate. In attribute upcast though, the range that is set there is incorrectly used/interpreted.

function _setAttributeOn( modelRange, modelAttribute, conversionApi ) {
	let result = false;

	// Set attribute on each item in range according to Schema.
	for ( const node of Array.from( modelRange.getItems() ) ) {
		if ( conversionApi.schema.checkAttribute( node, modelAttribute.key ) ) {
			conversionApi.writer.setAttribute( modelAttribute.key, modelAttribute.value, node );

			result = true;
		}
	}

	return result;
}

This function sets attributes on passed modelRange. The thing is that for elements, the range is like this: [<div>...</div>] not like this: [<div>]...</div>. So, in the given example, the range to set attribute border on is [0] - [1] instead of [0] - [0,0]. This means that all elements inside will also have that attribute set.

There are two solutions here: either we change _setAttributeOn function to work correctly for such ranges (changing in the loop to .getItems( { shallow: true } ) should be enough). Or we change what data.modelRange is generated for elements during conversion. I think that the first fix is probably safer.

BTW. we didn't notice that bug before because we don't have any element that can contain itself. So, whenever we set an attribute on an element it was not allowed on any of its children, so it worked.

[scofalik]

This was generated with shallow: true:

Seems like that fix would work.