Content with only "More" block reloads as invalid

[aduth]

Issue Overview

When saving a post with only the "More" block, reloading the post reveals multiple invalid blocks.

Steps to Reproduce (for bugs)

1. Navigate to Posts > New Post
2. Insert a More block
3. Save the post
4. Reload the page

Expected Behavior

The post is shown as it did prior to reload.

Current Behavior

The post is shown with multiple invalid blocks.

Possible Solution

This appears to be a conflict with autop behaviors. Specifically:

wpautop( '<!--more-->' ) === '<p><!--more--></p>';

The Gutenberg content parser treats this as three blocks:

• Freeform block with innerHTML <p>
• More block
• Freeform block with innerHTML </p>

The two freeform blocks are marked as invalid because reserializing will restore the missing closing / opening tags respectively.

A few options here:

• The "More" tag, if considered a proper block, could be exempted from wpautop as other blocks
• The grammar could be made more lenient to consider <p> wrapping tag for a More block
• The grammar could be made more strict to not consider More block if wrapped by <p>

[bobbingwide]

I got the same problem when selecting a post that was created in the Classic Editor. Switching to the Code editor reveals the changed source. There were no p tags in the original.

<p>Same problem happens when there's content.

<!--more-->

No line feeds here.</p>

Saving the above leads to even more confusion the next time the post is edited.

[aduth]

Related: #4005

Switching to the Code editor reveals the changed source.

With the above #4005, this will produce a slightly different result, but still not quite right:

<p>Same problem happens when there's content.</p>
<p><!--more--></p>
<p>No line feeds here.</p>

[mcsf]

Should be fixed by now (see #5061).