Infinite Scroll: Prevent first page duplication, and fix main query detection

[mikejolley]

I believe this fixes #1135 but we have slightly different reproduction steps in that we're simply running this on a custom post type archive. I found issues in the way pagination works which I'll do my best to explain below.

There are 2 main issues this PR aims to resolve;

1. Page "1" being loaded when you're already viewing page 1.
2. Queries from other plugins failing because the Infinite scroll query is not the "main query".

Changes proposed in this Pull Request:

Globals

This PR adjusts the order in which the globals wp_the_query and wp_query are set so that is_main_query is accurate when ran on the pre_get_posts hook.

Looking at the code where it set wp_the_query and wp_query it looks like the intention here was to be the 'main query' but because the query is RAN before these globals are set, any functions hooking into pre_get_posts will not see correct results.

Before:

• Query is ran in the constructor of WP_Query
• pre_get_posts fires during the query.
• At this point wp_the_query and wp_query have the original values. This means is_main_query will return false.

After:

• New WP_Query is constructed.
• wp_the_query and wp_query are set to this new WP_Query object.
• query is ran.
• pre_get_posts fires during the query.
• At this point wp_the_query and wp_query contain the current query object, so is_main_query will return true.

This fixes a conflict with WooCommerce which only adjusts the main query.

Pagination JavaScript

When viewing the first page and scrolling, IS loads page 1 again. The adjustment in the script I made makes it load page 1 plus the current page (offset).

So example, WooCommerce shop page at local.wordpress.test/shop/ is showing page 1 already. When you scroll it loads page 1 + offset 1 = page 2.

If you start at local.wordpress.test/shop/page/2 it loads page 1 + offset 2 + page 3.

Both the results are correct, and the address bar is updated accordingly.

I'm not sure how or if it actually worked before :)

Testing instructions:

So these are the testing instructions for our WooCommerce use case. @tiagonoronha May comment for general blog testing instructions but basically.

• Checkout this branch
• Checkout this branch of WooCommerce Infinite scroll compatibility woocommerce/woocommerce#19477
• In customizer > woocommerce > product catalog, set orderby to title, and show 3 products per row and 1 row per page.
• View the shop page.
• Scroll/load more, and you'll see another row of products added until complete.

Before the patch you'd see random number of products, in random order, with missing items in the grid. Page 1 would also be duplicated.

Proposed changelog entry for your changes:

• Fixes is_main_query detection within infinite scroll's WP_Query object.
• Fixes first page pagination to prevent duplicate pages being returned.

[danieldudzic]

I have tested this with Storefront (and few other themes), but I'm still experiencing an issue:

• Jetpack and WooCommerce branches checked out: https://cldup.com/vicAWyRaWq.png
• 18 published products: https://cldup.com/HUppNjd6dC.png (products .csv: https://cloudup.com/cLnPPDZgKus)
• With IS enabled: Shop page displays only 14 products (after scrolling to the bottom): https://cldup.com/-nupQRnSj1.png
• With IS disabled: Shop page displays 17 products: https://cldup.com/nKA4tiWVs0.png / https://cldup.com/ynGv6OuweR.png

[tiagonoronha]

@danieldudzic Thanks for testing.

Please test with #9120 instead. Also make sure that none of the themes has any custom code to alter the IS query. You can test with woocommerce/storefront#788.

[mikejolley]

^ +1 This patch alone won't make WC compatible.

[zinigor]

I have a small nitpick regarding the code, plus I seem to have problems when testing this PR. If you get a site with a big number of posts, you can observe for yourself:

• open a category view;
• scroll towards the second page;
• remember what posts you see;
• open the same link but in a new tab so that it has to load from the server;
• see that you don't get the same posts that were loaded in IS.

I can't put my finger on it, but I think that something is wrong with the offset, as though some posts are getting skipped because of posts per page number mismatch.

I'm happy to let you test on my site with tons of posts, just let me know if you need the creds. Here's a link to the site with IS and lots of posts with categories: https://dev71.lousy.site/category/cats/

[zinigor]

According to the PRs description, there's a pretty big change implied in this simple diff here. Do you think we should add a comment so that no one would have a desire to roll the thing back and avoid using an additional variable? :)

Or maybe we can even get rid of the variable, and just call the query method on one of the globals after they are set?

[zinigor]

After testing together with #9120 I can say that the problem is gone, well done, thank you!