Inconsistent handling of block equivalents of legacy widgets

[Frank Klein]

Assuming a site with a theme that supports block-based widget areas.

When using a widget block that maps to a legacy widget, WordPress will add additional classes on the frontend. This is handled through WP_Widget_Block::get_dynamic_classname().

As an example when using the Latest Posts widget, this is the markup on the frontend:

<div class="widget widget_block widget_recent_entries">
  <div class="widget-content">
    <ul class="wp-block-latest-posts__list wp-block-latest-posts">
      [...]
    </ul>
  </div>
</div>

The important part here is the widget_recent_entries.

However the same does not apply to the admin preview. Therefore the preview in the admin and on the frontend will always be different.

Two things need to happen here:

1. WordPress should be consistent about adding the backwards compatibility classes. This means that both the frontend and the backend preview need them.
2. There should be a filter to disable this backwards compatibility layer. Because not every theme developer will want this.

hen a theme uses block-based widget areas, then