Description

When going direct to a GB template edit URL e.g. /wp-admin/site-editor.php?p=%2Fwp_template%2Ftwentytwentyfive%2F%2Findex&canvas=edit a message saying "The theme you are currently using does not support this screen." briefly flashes up on screen and then disappears as the template is loaded.

This happens nearly imperceptibly when working on localhost with a nice clean WordPress install, but is more visible in real world conditions.

The message is rendered by <SidebarNavigationScreenUnsupported /> and is intended for classic themes.

I believe this is being displayed due to a race condition. It checks whether the current theme is a block theme before the current theme data has been loaded, and thus displays the message. Once the theme data is loaded it then doesn't display that component - it loads the editor as expected.

Step-by-step reproduction instructions

1. Have a block theme.
2. Go to /wp-admin/site-editor.php?p=%2Fwp_template%2Ftwentytwentyfive%2F%2Findex&canvas=edit
3. Keep a close eye

Screenshots, screen recording, code snippet

Around the 19 second mark

Environment info

• GB trunk (f8c6fa4 / 22.7.0+) with TT5
• Tested with Firefox

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

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure