What?

The JSON encoding with additional transformations causes some block attributes to be mis-encoded, resulting in invalid JSON and eliminating all the block attributes.

This is a partial fix, it also requires a fix to the core block serializer proposed in WordPress/wordpress-develop#9558. On its own, this fix prevents JSON from being mis-encoded on the client. However, the server continues to mis-encode JSON and lose the block attributes.

Coordinating the fixes correctly is a difficult challenge, however I think the fixes can be landed independently because if the issue persists in either the client or the server (Gutenberg or Core), the JSON will be mis-encoded and attributes stripped.

Block attribute string values (or keys, but this is much less common) that end in \ cause the transform to incorrectly replace a JSON string closing " with \u0022, breaking the JSON:

{
  "desired": "\\",
  "actual": "\\u0022
}

Fixes #16508.

Requires WordPress/wordpress-develop#9558 (Trac ticket 63917)

Why?

\ is valid string contents and should be allowed.

How?

Change the block attribute encoding to first replace escaped \ characters with its unicode escape sequence, then perform subsequent replacements. This ensures that " after and escaped \ character will not be incorrectly replaced. Additionally, this should make the JSON safe from wp_kses_stripslashes() because the problematic sequence \" should never appear in the JSON.

Change the escaping regular expression to ensure that the escaped " character (\") is not a false escape that is actually preceded by an escaped \ character like \\".

Testing Instructions

Create a new post at /wp-admin/post-new.php.
Add a "More" block (/more).
Replace the default text with the backslash character: \.

Save. Before this patch block would have invalid attributes. Checking the code editor reveals this:

<!-- wp:more {"customText":"\\u0022} -->
<!--more \-->
<!-- /wp:more -->

With this patch, the block attributes are encoded as valid JSON and the \ custom text is preserved in the block.