Template parts do not load in the full-site editor.

[kjellr]

Describe the bug

Template parts that had previously been loading fine using Gutenberg 9.3 are no longer loading in 9.4.

To reproduce

1. Activate a block-based theme that uses block template parts.
2. Visit the full-site editor.
3. Check to see if the template parts load.

Screenshots

Tested with Gutenberg 9.4, and a handful of block-based themes (Twenty-Twenty One Blocks, Seedlet Blocks, etc.)

[kjellr]

cc @noahtallen since you noted something similar in #26383 (review)

[ockham]

As always, let's add some e2e coverage for this when fixing (however minimal -- just to make sure this stuff loads).

[kjellr]

Upon further testing: I'm still able to reproduce this on all of my usual local test sites, using both master, and 9.4-rc1. But if I spin up a fresh, live testing site it seems to work ok. 😩 There's a possibility it's just a me problem, though I've heard similar reports in other contexts lately.

I'm curious if others can reproduce.

[jffng]

I can reproduce this locally on master.

The requests for the template parts return a 200 status, but the response is empty.

Please let us know if there is other info we can provide to help debug, thank you!

edit

Some additional context:

I've tested using Seedlet blocks, Mayland blocks, and Monticello blocks all with the same result. Fresh WP 5.5.3 instance and running master / 9.4.0.

Something that I'm unfamiliar with and am not sure if it's related is that "auto-drafts" are automatically created when I switch to a block based theme and view the templates from wp-admin:

[noahtallen]

I came across this here: #26383 (review) and thought it was an issue in the PR. I guess folks thought it got fixed before merging? Could also be somewhere else, but that was the only place I noticed it.

[vindl]

This is a result of #27016. We changed the way of tagging templates with themes (from meta to taxonomies), and they no longer work with old sites created with 9.3. This can be resolved by cleaning the CPTs locally. Since FSE is experimental feature we are not planning on providing migration code in Gutenberg to fix this automatically.