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.