Forms: fix telephone country dropdown when selected

[CGastrell]

Proposed changes:

This PR fixes a wee issue seen on some themes when the country dropdown is shown on the telephone field. The dropdown can show behind other content. The fix raises the z-index to 1 on the container div.

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After:

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?
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Jetpack product discussion

None

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

No

Testing instructions:

Insert a telephone field block, open the country selector dropdown. Verify that it renders on top of other content.

[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/forms-field-telephone-dropdown-z branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/forms-field-telephone-dropdown-z

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[github-actions]

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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).
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  • Scheduled release: October 7, 2025
  • Code freeze: October 6, 2025

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[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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 .}

[simison]

Haven't tested but looks alright