Site Editor: Unintended title display and content width

[t-hamano]

Most canvases in the site editor seem to be experiencing the following issue.

• The entity title is displayed
• The content width is narrow

When I searched for the problematic commit using git bisect, #58935 seems to be affected. #58935 has a backport label, so it may need to be fixed soon.

Actual

Expected

[glendaviesnz]

@youknowriad the post-only mode is being used to apply some post specific layout, eg the post title and some of the global styles layout widths, eg. here, so applying post-only to all entities is what is causing this issue.

[youknowriad]

Looking into this.