What problem does this address?

In FSE Templates, we are allowed to add custom classes to the Post Content block. However, the same classes are not applied on the .is-root-container.wp-block-post-content in the Block Editor.

Having these classes show up would help with styling different templates within the Editor. For example, the default block gap might change for a template that is more Cover block and Columns rather than one that is more a news article with Headlines and Paragraphs.

What is your proposed solution?

Have the custom classes assigned in the Post Content block of a template be applied in Block Editor.