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

---

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Props westonruter, mukesh27.

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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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