Block editor preview not displaying unsaved changes to a post when meta boxes are active

[talldanwp]

Originally created as a github issue, for background see here:
​https://github.com/WordPress/gutenberg/issues/12617

The cause of this is explained in this comment:
​https://github.com/WordPress/gutenberg/issues/12617#issuecomment-452604656

1. A published post is previewed, generating an autosave.
2. The POST request to save PHP meta boxes is sent to post.php. The published post is updated and its last-modified time is bumped.
3. The POST request is redirected as a GET request to post.php again. This logic runs, which compares the modified dates of the autosave and the post: ​https://github.com/WordPress/wordpress-develop/blob/e5b5db9e2349dfe8a43ac42bea5738146f53994d/src/wp-admin/edit-form-blocks.php#L294-L303
4. The modified date of the post is now more recent than the autosave, so the autosave is deleted.
5. The preview loads, but there's no more autosave to preview.

To test:

1. Create a new post using the block editor
2. Open the options dialog and enable 'Custom Fields'
3. Add a title, content and a custom field to the post and save it
4. Add some further changes to the post content and click the preview button

Expected behaviour
Changes added in step 4 are displayed in the preview

Actual behaviour
None of the changes are visible in the preview

[dlh]

Related: #45768.

[pento]

Closing this, as it was ultimately fixed in GB11409.

The larger issue of supporting post meta revisions is tracked in #20564.

[earnjam]

I think you meant fixed in GB14877?

[pento]

Yep, thanks for noticing that, @earnjam!