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• Have you followed the WooCommerce Contributing guideline?
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Changes proposed in this Pull Request:

In #20640 it was reported that the trigger to mimic a country select was causing issue, it was then removed which broke the field priority on intial page load. #21733 added back an onselect trigger without knowing it will cause #20640 to happen again.

This PR removes the onselect trigger again and goes with another approach by sorting via PHP, thanks @pierrebuet for the suggestion and idea based on the code you provided. I reworked Pierre's code a bit as it was not working on all the filters you can use to change the checkout fields and also refactored the other uasort methods we have by introducing a new wc_uasort_comparison method which can be reused.

Closes #21642

How to test the changes in this Pull Request:

1. Use either the woocommerce_checkout_fields or the woocommerce_default_address_fields or any of the other filters you can use to alter the checkout fields and changes the priority of one of the fields to 1 so it should show right at the top of the checkout page.
2. Load the checkout page and ensure the field is displayed at the correct place based on the priority set.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

Changelog entry

Fix - Checkout field priority was not applying on initial page load.