Fix AI Assistant modal shaking when content streams in

[CGastrell]

Part of JPAI-95

Fixes a visual issue where the AI Assistant modal would shake/jitter as content streamed in.

Proposed changes

• Fix modal header spacing (ai-client): The header used -32px negative margins that didn't match the WordPress Modal component's actual 24px content padding, causing layout instability as content changed. Corrected to -24px.
• Make header sticky (ai-client): Replace the separate <hr> divider element with a border-bottom on the header and add position: sticky so the header stays in place when content scrolls, reducing visual jumpiness.
• Align consumer components (plugins/jetpack): Update the feedback suggestion and title optimization footer components to use matching -24px margins/padding.

Screen.Recording.2026-03-16.at.17.44.33.mov

Other information

• Generate changelog entries for this PR (using AI).

Related product discussion/links

p1773669718127489-slack-CDLH4C1UZ

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

No.

Testing instructions

1. Open a post in the block editor
2. Open the AI Assistant from the Jetpack sidebar
3. Use "Optimize title" or request feedback on the post
4. Observe the modal as text streams in — it should no longer shake or jitter
5. If the modal content is long enough to scroll, verify the header stays pinned at the top

[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/ai-modal-spacing-and-sticky-header branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/ai-modal-spacing-and-sticky-header

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]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add changelog entries to affected projects
  

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Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!

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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: March 31, 2026
  • Code freeze: March 31, 2026

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

[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 .}