Ensure IMG with fetchpriority=low does not get lazy-loaded, increase media count, or interfere with assigning fetchpriority=high

[westonruter]

Trac ticket: https://core.trac.wordpress.org/ticket/64823

With WordPress/gutenberg#76208 checked out and there are 5 large images added to a Navigation Overlay and 5 large images in the post content, given the following script being run in the console:

Array.from(document.querySelectorAll(".wp-site-blocks img")).map((img) => {
  return {
    insideNavOverlay: !!img.closest(".wp-block-navigation__overlay-container"),
    loading: img.loading,
    fetchPriority: img.fetchPriority,
  };
});

Before ❌

[
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    }
]

After ✅

[
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": true,
        "loading": "auto",
        "fetchPriority": "low"
    },
    {
        "insideNavOverlay": false,
        "loading": "auto",
        "fetchPriority": "high"
    },
    {
        "insideNavOverlay": false,
        "loading": "auto",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "auto",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    },
    {
        "insideNavOverlay": false,
        "loading": "lazy",
        "fetchPriority": "auto"
    }
]

The diff shows the changes to the first three images in the content (outside of the navigation overlay):

--- before.json	2026-03-06 15:32:18
+++ after.json	2026-03-06 15:32:27
@@ -26,17 +26,17 @@
     },
     {
         "insideNavOverlay": false,
-        "loading": "lazy",
-        "fetchPriority": "auto"
+        "loading": "auto",
+        "fetchPriority": "high"
     },
     {
         "insideNavOverlay": false,
-        "loading": "lazy",
+        "loading": "auto",
         "fetchPriority": "auto"
     },
     {
         "insideNavOverlay": false,
-        "loading": "lazy",
+        "loading": "auto",
         "fetchPriority": "auto"
     },
     {

Fixes:

1. Preserve fetchpriority=high being assigned to the first large image outside of the Navigation Overlay.
2. Prevent adding loading=lazy on the first three images in the content.

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• Copilot for review

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[github-actions]

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[westonruter]

cc @mukeshpanchal27

[Copilot]

Pull request overview

Updates core loading-optimization heuristics to treat img[fetchpriority=low] as a special case so it won’t be lazy-loaded, won’t affect the content media count, and won’t interfere with assigning fetchpriority=high to the first in-viewport large image.

Changes:

• Extend wp_get_loading_optimization_attributes() to recognize existing fetchpriority=low and avoid adding loading="lazy" / incrementing media count.
• Add PHPUnit coverage to ensure fetchpriority=low images don’t influence the “first N images not lazy-loaded” and high-priority selection logic.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/wp-includes/media.php	Adds fetchpriority=low handling in loading optimization logic and documents the behavior.
tests/phpunit/tests/media.php	Adds helper + additional assertions to cover fetchpriority=low interactions across contexts/loops.

---

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[mukeshpanchal27]

Suggested change

	"Expected first image to not be lazy-loaded. First large image gets high fetchpriority."
	'Expected first image to not be lazy-loaded. First large image gets high fetchpriority.'

[mukeshpanchal27]

Suggested change

	$maybe_in_viewport = false;
	$maybe_in_viewport = false;
	$loading_attrs['fetchpriority'] = 'low';

Shell we add here instead of separate condition?

// Preserve fetchpriority=low.
if ( $is_low_fetchpriority ) {
	$loading_attrs['fetchpriority'] = 'low';
}