Previewing the page designated as "latest posts" shows the frontpage

[zieladam]

This continues the discussion from ​https://github.com/WordPress/gutenberg/issues/2409

The issue

• Create two pages (let's call them "My home" and "My posts")
• Go to customizer and set Homepage to be "My home" and Posts page to "My posts"
• Edit "My posts" in editor mode
• Press preview
• Confirm you got "My home" instead of "My posts"

The root cause

When you click "Open preview in new tab" while editing "My posts", Gutenberg redirects to a preview URL like this one:

​https://mywpsite.com/?page_id=5&preview_id=5&preview_nonce=12bd60d6f4&preview=true

When you visit that URL, WordPress will load the front page instead of the posts page. This is because class-wp-query.php assumes that posts page is also the front page:

​https://github.com/WordPress/WordPress/blob/b4373fafe9b87f75bf9d65e808be8049510dff8b/wp-includes/class-wp-query.php#L1032

Then, when rendering a preview, it substitutes the page_id that was requested with the value of get_option( 'page_on_front' ):

​https://github.com/WordPress/WordPress/blob/b4373fafe9b87f75bf9d65e808be8049510dff8b/wp-includes/class-wp-query.php#L1904

If I remove the preview parameters and leave only ?page_id=5, it displays the correct page.

[joyously]

As stated in the GitHub issue, that issue was only a continuation of #34005

[zieladam]

In my understanding, #34005 is about re-enabling the Visual Editor and Page Template dropdown when editing the page for Posts. The discussion in the Gutenberg issue ​https://github.com/WordPress/gutenberg/issues/2409 pivoted towards the results of previewing the page marked as "Posts page" - which indeed is unexpected due to a bug. I believe this constitutes it's own issue.