Fix button block contrast checker

[talldan]

Proposed changes:

Fixes an issue with the Jetpack Button block's contrast checking. Currently the contrast checker only works when the user has set both the background and text color.

If the user changes only one of the text or background color in a way that causes a contrast issue, the checker doesn't display a warning.

Some history - this functionality was previously handled by a higher order component - applyFallbackStyles / withFallbackStyles. #41139 removed the HOC because it had a few issues:

• It added a wrapping <div> around the block which breaks a range of styles
• It didn't check the correct element for background colors
• It didn't update when the block's css changes (e.g if inline styles or a class are added).

This PR introduces a small hook to replace the applyFallbackStyles / withFallbackStyles to keep the contrast checker working.

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)?

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

No

Testing instructions:

1. Add a contact form block
2. Select the button in the form
3. Change only the text color to something that conflicts with the button background (e.g. if there's a black background, set the text to black). Or the opposite, change only the background color.
4. Observe the contrast checker should be displayed. (in trunk it doesn't show).

[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/button-fallback-contrast-checking branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/button-fallback-contrast-checking

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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---

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  • Scheduled release: April 1, 2025
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[github-actions]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/extensions/blocks/button/edit.js	9/10 (90.00%)	1.11%	0 💚

1 file is newly checked for coverage.

File	Coverage
projects/plugins/jetpack/extensions/blocks/button/use-fallback-colors.js	7/12 (58.33%) 💚

Full summary · PHP report · JS report

[aaronrobertshaw]

Thanks for squashing this bug @talldan 👍

I had to rebase this branch for it to build locally for me but once it did everything tested as advertised.

✅ Warning displayed when poorly contrasting colors were chosen
✅ Warning worked regardless of when the combination was user selections, inherited values etc.
✅ Warning was removed when color combination improved or reset to valid values

With the push to improve test coverage in Jetpack, could we add some tests for the hook here as well?

I also left a tiny optimization suggestion as an inline comment. Other than that I think this is looking pretty good.

Screen.Recording.2025-02-28.at.5.18.17.pm.mp4

[aaronrobertshaw]

I think we can make a small optimization here by avoiding repeated calls to getComputedStyle( node ).

[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/extensions/blocks/button/edit.js	9/10 (90.00%)	1.11%	0 💚

1 file is newly checked for coverage.

File	Coverage
projects/plugins/jetpack/extensions/blocks/button/use-fallback-colors.js	14/15 (93.33%) 💚

Full summary · PHP report · JS report

[talldan]

Thanks for the review. Feedback should all be addressed 👍

[aaronrobertshaw]

Nice work @talldan ✨

This is testing well for me. I couldn't break the contrast checker and the tests all pass.

LGTM 🚢