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Agree with @ironprogrammer, code looks good. The only thing I'm a bit unsure about is the part of site-editor.php that deals with template parts and non-block themes: https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-admin/site-editor.php#L22-L35.

If the template parts menu item is removed, does that code need updating too?

Thanks for the review!

The only thing I'm a bit unsure about is the part of site-editor.php that deals with template parts and non-block themes: https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-admin/site-editor.php#L22-L35.

If the template parts menu item is removed, does that code need updating too?

The code here, i.e. $is_template_part_editor, should be maintained for backward compatibility. This is because the path (wp-admin/site-editor.php?path=/wp_template_part/all ) may have been hard-coded by the developer or added to the browser's bookmarks by the user.
On the site editor side, when this old path is accessed, it redirects to the pattern page. See WordPress/gutenberg#61446 for details.

should be maintained for backward compatibility.
...may have been hard-coded by the developer or added to the browser's bookmarks by the user.

Right, that's why I asked :)

On the site editor side, when this old path is accessed, it redirects to the pattern page.

Yep, this is how it should be. However it doesn't seem to redirect in WP core when testing this PR (unless I'm not testing it right?). It goes to a "Manage template parts" screen which of course doesn't work right. Thinking it would be good to have that redirect in PHP too.

Here's a screenshot with this PR applied and testing the old URL:

However it doesn't seem to redirect in WP core when testing this PR

I think it's because the latest Gutenberg hasn't been merged into core yet, although JS redirection is implemented here.

I feel that if we implement PHP redirection in the core, JS redirection in Gutenberg might be unnecessary.

cc @youknowriad @jsnajdr

I feel that if we implement PHP redirection in the core, JS redirection in Gutenberg might be unnecessary.

Yes, thinking so too. Generally PHP redirects are preferable as they run faster (the browser stops loading the current page and redirects as soon as it receives the header of the response).

I think JS redirection is probably necessary here because the site editor is an SPA and folks (third-parties) can navigate to URLs without actually reaching the server.

With changeset 58187, The JS side redirect is now reflected in the core. Do we also need a PHP redirect just in case?

I don't think it's needed personally. This PR seems ready to ship for me.

Checking WP Directory for use of \/wp_template_part\/all, of the [only] two hard-coded links in those results, they redirect to site-editor.php?path=/patterns as expected. I agree this is good to go.

Committed in https://core.trac.wordpress.org/changeset/58278