Boost: Align Cloud response with plugin for BG Images

[LiamSarsfield]

Note

Merge this PR after #43420 is merged.

Resolves HOG-108: Boost Plugin: Align Cloud response with plugin for BG Images

Proposed changes:

• This PR supports background images being the LCP for a specific page.
• It ensures that the preloaded background image is an appropriate responsively set image, while also applying media queries for the viewports we support once the page has fully loaded.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Boost: Improve visitor experience by delivering key images faster - better LCP

Does this pull request change what data or activity we track or use?

N/A

Testing instructions:

Prerequisites

• Ensure that https://github.com/Automattic/boost-cloud/pull/680 is checked out locally.
• Latest Boost Developer plugin is installed, and LCP Type is set to Background Image.
• At least one Cornerstone Page set.
• Boost Feature Flag enabled within the install (JETPACK_BOOST_ALPHA_FEATURES)
• LCP Optimization feature enabled.
• Image Size Analysis disabled.

Apply the following snippet below to test for multiple responsive changes of the LCP Background Image.

Width Changes

add_action('wp_head', function() {
  ?>
  <style>
      #jb-test-lcp {
          width: 100%;
          max-width: 800px;
          height: 400px;
          margin: 0 auto 20px;
          display: block;
          border: 1px dashed #ccc;
          position: relative;
      }

      /* Grows to 360px at 360px viewport */
      @media (min-width: 360px) {
          #jb-test-lcp {
              width: 360px;
              height: 200px;
          }
      }

      /* Grows to 400px at 440px viewport */
      @media (min-width: 440px) {
          #jb-test-lcp {
              width: 400px;
              height: 225px;
          }
      }

      /* Full width at 768px viewport */
      @media (min-width: 768px) {
          #jb-test-lcp {
              width: 768px;
              height: 432px;
          }
      }

      /* Becomes 600px width at 1024px viewport */
      @media (min-width: 1024px) {
          #jb-test-lcp {
              width: 600px;
              height: 337px;
          }
      }

      /* Becomes 400px width again at 1366px viewport (same as 440px) */
      @media (min-width: 1366px) {
          #jb-test-lcp {
              width: 400px;
              height: 225px;
          }
      }

      /* Full width again at 1920px viewport */
      @media (min-width: 1600px) {
          #jb-test-lcp {
              width: 100%;
              height: 1080px;
          }
      }
  </style>
  <?php
});

1. Enable up Jurassic Tube/Localhost tunnel and login to the public URL available.
2. Click Optimize on the LCP Feature and wait for it to be completed.
3. Once completed, open your network tab, and go to a Cornerstone Page.
4. Filter the network tab, and find the LCP image being fetched. You should notice the Photon CDN URL, with an appropriate width of the image.
5. Ensure that the image was not requested twice for different widths, as this indicates the BG Media Query, or the preloading is not applying the correct image.
6. Change the viewport from the dev tools, you should see the LCP image BG URL width is changing in relation to the actual width of the image itself.
7. When a new width is being requested, refresh the page to ensure that the same width is being requested by the preloading, and it's not being changed once the loading has been finished.

[github-actions]

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---

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---

Boost plugin:

• Next scheduled release: May 29, 2025

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[jp-launch-control]

Code Coverage Summary

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If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, this should be updated once coverage for dcb8a8b is available.

Full summary · PHP report · JS report

_{Coverage check overridden by Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR .}