FSE: All blocks show up as full width, don't match the post editor.

[kjellr]

It's quite likely there's another issue for this somewhere, but I couldn't find it.

In the Full Site Editor, all blocks show up as full width.

Here's a comparison of a simple post in the post editor vs. the full-side editor, using the Twenty Twenty-One Blocks theme. The theme is providing no alignment overrides at all in the back end.

Post Editor	Site Editor

This has been the case for as long as I can remember — how can we fix that? Does the site editor just need alignment CSS to match the post editor?

Related: #25160

[kjellr]

I was able to solve this for Twenty Twenty-One blocks in WordPress/theme-experiments#80. It seems that the problem is that Gutenberg doesn't provide max-widths for blocks by default in the full-site editor.