Slideshow block: ensure image size selection works correctly

[coder-karen]

Fixes VULCAN-310

Proposed changes:

• This PR prevents an issue where image size selection for a Slideshow block (thumbnail, medium, large or fullsize) is not always correctly reflected in the editor. When saved in that state, the same state appears to transfer to the front-end as well.

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

n/a

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

Testing instructions:

To replicate the issue:

• On a self-hosted, WoA or Simple test site, add a new Slideshow block with multiple images to a post.
• Wait for the post to fully load, then change the image size. This may not be easy to replicate, but you'd be replicating the issue if the image size does not change in the editor.
• If you can replicate the issue consistently, then you will be able to test the fix. Otherwise, confirm that slideshow image size changing is not affected by the changes.

To test the fix:

• Apply this PR and follow the same steps as above. When changing image sizes via the sidebar, this should be reflected in the editor. When saving the post, this should be visible on the front-end as well.

Note for Simple: Thumbnail size may not be 150 x 150 - this is not related to the Slideshow block. You can confirm by adding an image block and setting the resolution to thumbnail - the image thumbnail will look the same.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the fix/slideshow-block-image-size-selection branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/slideshow-block-image-size-selection

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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Follow this PR Review Process:

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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen as soon as you deploy your changes after merging this PR (PCYsg-Jjm-p2).
• WoA releases happen weekly.
• Releases to self-hosted sites happen monthly:
  • Scheduled release: November 4, 2025
  • Code freeze: November 3, 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

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/extensions/blocks/slideshow/edit.js	22/57 (38.60%)	0.89%	-3 💚

Full summary · PHP report · JS report

[fgiannar]

Thanks for working on this, Karen!

I was able to reproduce the bug and confirm that the fix works with an exception when transitioning from anything to "Large". For example I tried switching from "Large" to "Thumbnail" and it works well. But then when I switched again to "Large" nothing happened. Can you also reproduce this?

[coder-karen]

Thanks for testing!

No, I can't reproduce that. Which environment were you testing in?

[fgiannar]

Thanks for testing!

No, I can't reproduce that. Which environment were you testing in?

Testing with a WoA dev site, apologies for not specifying this in the first place.

[coder-karen]

Noting here that the issue was the image sizes used on the test site - the image sizes on the test site were only 700 x 700px, and the large image size as defined in media settings on the site and most sites is set by default to 1024 x 1024. I would imagine this was probably the same behaviour before but can't confirm that - the code would suggest so. In that case it makes sense - no 'large' image size actually exists for that image, so there is nothing to select.
But we can add a fallback for those cases - falling back to 'full size' if no size is defined. Added in 111c963