Relative scheme images no longer display in the editor

[peterwilsoncc]

Description

In WordPress 6.3 and the current version of the Gutenberg plugin, 16.6.0, images with relative schemes are not displayed correctly in the block editor. On the front end they render as expected.

In the editor, the images are shown as a broken image and -- browser dependent -- the alt text shown in it's place.

Step-by-step reproduction instructions

1. Install WordPress 6.3 locally
2. In the wp-config file, define a relative content URL constant: define( 'WP_CONTENT_URL', '//example.local/wp-content' );
3. Create a post in the block editor
4. Insert an image block
5. Upload an image to the image block/library
6. The image fails to load
7. Preview the post in a new tab: the image does display
8. Switch to the code editor
9. Modify the image URL to include the scheme
10. Exit the code editor
11. The image now displays

Screenshots, screen recording, code snippet

WordPress 6.3 (with and without the latest version of Gutenberg active)

WordPress 6.2 (without Gutenberg active)

Environment info

• WordPress 6.3
• Browsers: Reproduced in Firefox, Safari and Chrome (all latest versions)
• OS: macOS
• Virtual machine: https://github.com/chassis/chassis on VirtualBox
• Home URL: example.local
• Site (WP) URL: example.local/wp (ie WordPress is running in its own sub-directory)

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[tambourine-man]

Same here, basically makes using relative path in WP_CONTENT_URL impossible.

[Pat-Relentless]

Also have this issue if you define a gutenberg template for a post type that uses an image:

"template": [
[
	"core/cover",
	{
		"url": "/wp-content/uploads/2023/11/image.jpg",
		"id": 38,
		"dimRatio": 30,
		"overlayColor": "forest-green",
		"layout": {
			"type": "constrained"
		}
	},

I suspect this has something to do with the editor being rendered in an iframe where the src starts with "blob". Likely a security issue, but would love to be able to use relative image urls in a block template...