Infinite Scroll: No longer treated as an ajax request

[WPprodigy]

Infinite Scroll requests are no longer being treated as ajax requests, causing various pre_get_post filters to incorrectly alter the query - breaking things pretty bad in some cases.

IS doesn't use a traditional WP ajax flow (unsure why?), but attempts to tell WP it is an ajax request here: https://github.com/Automattic/jetpack/blob/master/modules/infinite-scroll/infinity.php#L1725

However, with the latest changes to IS (presumably the jQuery removal), this no longer happens because HTTP_X_REQUESTED_WITH is never set.

Replicated with JP version 8.5 and 8.4.2. Does not happen in 8.3.

One solution is to alter the PHP side and just see if the request method is POST (cleanest option?). Otherwise can continue passing XMLHTTPREQUEST like jQuery.ajax does around here: https://github.com/Automattic/jetpack/blob/master/modules/infinite-scroll/infinity.js#L353 (probably best for BC?)

[jeherve]

cc @dero who worked on the Infinite Scroll changes in #15017

[dero]

@WPprodigy @jeherve Thanks for bringing this to my attention, it's clearly a regression. Created a PR that ensures the X- header is sent along with the AJAX request even in vanilla JS.

• Infinite Scroll: Provide the 'X-Requested-With' header #15843