Fix negative offset on hierarchical post types

[tfrommen]

Description of the Change

This PR ensures that the from argument (offset) for post search requests is not negative. This will happen on hierarchical post types on pages other than the first.

This is because ElasticPress uses posts_per_page to calculate the offset, POSTS_PER_PAGE * ( PAGE - 1 ). For hierarchical post types, however, this value is overwritten to -1 by WordPress (most probably to fetch all posts so that posts to be shown on the current page that have a parent can be listed with their parent post).

Alternate Designs

n/a

Benefits

Working pagination on hierarchical post types such as the default page. 🎉

Possible Drawbacks

n/a

Verification Process

1. Go to Pages in the admin: /wp-admin/edit-php?post_type=page
2. Search for a word or phrase where you have lots of posts that include this.
3. Use pagination buttons to navigate to, say, page 3.
4. Without this change, this will result in a request error for Elasticsearch. With this change, everything is working as expected, and you can navigate to any page you like.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

#2480

Changelog Entry

• Fix broken search pagination on hierarchical post types