Site Editor: Template part is missing after exiting focus mode

[Mamaduka]

When switching into focus mode to edit the template part, it's missing after going back to the template using the "Back" button.

Step-by-step reproduction instructions

1. Go to Appearance > Editor
2. Open templates details dropdown in the header.
3. Click on "More Options" for the area and "Edit {templatePartName}
4. Without doing anything, click the "Back" button.
5. See that now the template part you were editing is missing from the page.

Screenshot

CleanShot.2021-11-28.at.23.55.06.mp4

I tested with TT1 Blocks and TT2 themes.

[Mamaduka]

@kevin940726, I remember you worked on this feature. Any ideas why this might be happening?

[kevin940726]

I did the bisecting and found the first bad commit is coming from #36340. I have no idea why though. The problem goes away if I commented out this part:

5458edc#diff-884dae78f1676d454c049107e9b05b2403eb042457f77f836bd170d104396b8bR226-R233

Maybe @tellthemachines knows why?

[Mamaduka]

Thanks for the follow-up, @kevin940726.

[tellthemachines]

The problem goes away if I commented out this part:
5458edc#diff-884dae78f1676d454c049107e9b05b2403eb042457f77f836bd170d104396b8bR226-R233

No idea, but that code's specific to the navigation block. Does this issue happen with any template part or only one that contains a navigation?

[tellthemachines]

Answering my own question above 😅, I can reproduce this on latest trunk with any template part, whatever it contains. I'm also seeing a few PHP notices when loading the editor, not sure if they may be related:

Notice: Undefined property: WP_Block_Template::$origin in /var/www/html/wp-content/plugins/gutenberg/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php on line 467

Notice: Undefined property: WP_Block_Template::$author in /var/www/html/wp-content/plugins/gutenberg/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php on line 477

[Mamaduka]

Thanks for looking at the problem, @tellthemachines.

The PHP notices should be fixed now, but I don't think those are the cause.

[noisysocks]

The symptoms here look suspiciously like the ones in #36788. Could be related.

[Mamaduka]

Hi folks, I tried to debug this issue but failed 😞

Here are my findings so far:

• Started happening in Gutenberg 12.0
• The template part block's Edit component is rendered, but inner blocks aren't.
• We've all props required to render inner blocks, but they refuse to do so.
• When testing with TT2, I remove Site Logo and Navigation blocks from the header. Go back works as expected.

[noisysocks]

Pinging @WordPress/gutenberg-core for technical feedback.