There is a flash when clicking the template name in the editor if a plugin registered template matches a default WP theme template

[aljullu]

Same as this GitHub issue: ​https://github.com/WordPress/gutenberg/issues/65584

Steps to reproduce:

0. Assuming you are using a theme with a page.html template (ie: Twenty Twenty Four).
1. Add this code snippet to your site:

<?php

add_action( 'init', function() {
        register_block_template(
                'templates//page',
                [
                        'title' => 'My Single Page',
                        'description' => 'This is my single page template',
                        'content' => '<!-- wp:paragraph --><p>This is a plugin-registered template.</p><!-- /wp:paragraph -->',
                        'post_types' => [ 'page' ],
                ]
        );
} );

2. Go to the New page screen.
3. Click on the Pages button next to the Templates label.
4. Notice a flash.

This PR includes the changes that need to be backported from ​https://github.com/WordPress/gutenberg/pull/66359 into WC core.

Trac ticket: https://core.trac.wordpress.org/ticket/62319

Thanks for the review, @apermo! Would you mind taking another look? I added a couple of commits that fix some edge cases and add unit tests.

Thanks for the approval, @apermo! I don't have merging rights, is that something you can take care of?

Otherwise, @azaozz, wondering if this is something you can do? Thanks in advance!

Thanks for the approval, @apermo! I don't have merging rights, is that something you can take care of?

I can't merge either, this happens on the subversion by those with commit right(core commiters), I just reviewed.

[azaozz]

Seems this has been in Gutenberg for a while and needs adding to core.

This looks good, but I'd appreciate some more eyes maybe from @anton-vlasenko or @felixarntz ?

Thanks for the review, @ntsekouras! I answered/applied the feedback.

@felixarntz I don't have permissions to merge into WordPress, is that something you or somebody else can take care of? :pray:

I see there are a couple of checks failing, but they seem unrelated to this PR.

I tested the PR and it seems to fix the issue:

Before:
[Uploading before-applying-the-PR.webm…]()

After:

Before:
​before-applying-the-PR.webm

After:
​after-applying-the-PR.webm

I tested the PR and it seems to fix the issue:

Before:
[Uploading before-applying-the-PR.webm…]()

After:

@Aljullu I tested the fix too, together with the code snippet you posted on the ticket:

add_action( 'init', function() {
        register_block_template(
                'templates//page',
                [
                        'title' => 'My Single Page',
                        'description' => 'This is my single page template',
                        'content' => '<p>This is a plugin-registered template.</p>',
                        'post_types' => [ 'page' ],
                ]
        );
} );

The flash is fixed, and also there was actually another problem before where the "Use default template" button would actually not change the template content displayed. So both of these issues are resolved with this PR.

However, I do not see the custom "My Single Page" template listed now at all. Using the Twenty Twenty-Five theme, when I click "Change Template", I only see the built-in "Page No Title" template, but not the custom one from the above snippet. Without this PR applied, that custom template _does_ show up. So it seems while this PR fixes the problem, it introduces another one?

Can you check on your end please?

[flixos90]

Both @audrasjb and I tested the fix and left feedback on the PR. Everything looking good.

[flixos90]

In 59951:

Editor: Fix unexpected behavior due to conflicting custom block template.

This changeset fixes both a visual and functional bug related to template selection in the editor that occurred when having a custom block template registered that was using the same slug as another block template already registered by the theme, including the default block templates.

Props aljullu, antonvlasenko, apermo, audrasjb, azaozz, ntsekouras.
Fixes #62319.