drupalmedia plugin.js doesn't delete attributes from drupal-media element when unset.

Problem/Motivation

DrupalMedia plugin's plugin.js has this code:

this.element.setAttributes(this.data.attributes);

This unfortunately doesn't unset attributes such as 'data-align' as one would expect. If in the dialog, you set 'data-align' to 'none', one expects that it's removed from the embed. While it does remove it from the downcast embed, it doesn't remove it from the drupal-media element inside the widget. It doesn't unset previously set attributes.

This is a blocker for #3072279: Stop generating align-* classes for preview in DrupalMedia CKEditor plugin

this.element is a CKEDITOR.dom.element:

window.CKEDITOR.window.CKEDITOR.dom.CKEDITOR.dom.element {$: drupal-media.cke_widget_element}
$: drupal-media.cke_widget_element
...
attributes: NamedNodeMap
0: data-cke-widget-data
1: data-cke-widget-upcasted
2: data-cke-widget-keep-attr
3: data-widget
4: class
5: data-view-mode
6: data-caption
7: data-entity-type
8: data-entity-uuid
9: data-align
...

Proposed resolution

I think we need to iterate through the old data attributes and remove them (this.oldData.attributes in drupalmedia plugin.js)

If you try this.element.removeAttributes() and then set the attributes, CKEditor blows up, because some of the attributes are internal to CKEditor.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet