Site Editor: Content-width and wide widths sometimes don't apply when browsing through Pages

[jasmussen]

This one is hard to describe, so it's best to describe steps to reproduce:

• Have some pages saved/published on your site.
• Have a site design, ideally a Header template part, that leverages the wide width for the main containing group. E.g. Twenty TwentyFour by default.
• Go to the site editor (/wp-admin/site-editor.php).
• Navigate to "Pages"
• Select a page, and observe how the wide content-width for the header template becomes full-wide (or just unbounded).
• Now reload the page and observe how upon reload, the same wide content-width for the header template is correct.

The above steps are shown in these two GIF files as well:

In other words, the wide width is not respected when browsing to a page, that incorrect width even persists inside the editor. Until you reload in either the site editor page detail page, or the fullscreen editor. So some width-related CSS is missing in one case, but not the other.

[jasmussen]

Testing this more, here are the results:

• 6.4.3: not affected by this issue.
• 6.4.3 with the trunk Gutenberg plugin: can reproduce.
• 6.5 nightly: can reproduce. (With or without the Gutenberg trunk plugin.)

[andrewserong]

I'm not too sure the cause, but from a (very) quick test, it looks like the .has-global-padding rules aren't being output (potentially) when we go to view the individual page. However, upon reloading the site editor, these rules are applying correctly:

So in terms of next steps to investigate, I wonder why the global padding rules might be being skipped or not being output correctly in this case?

I might not get a chance to dig any deeper until later on in the week since I'm travelling.