My Jetpack: fix Overview tab scrolling under fixed wpbody-content

[CGastrell]

Fixes the scroll-past-the-end regression on the My Jetpack Overview tab introduced (visibly) by the recent layout-mixin adoption (#48503), and adds a defensive guard in the layout mixin so the same trap can't bite future consumers.

Proposed changes

• My Jetpack: drop overflow-x: hidden on the <AdminPage> root. .my-jetpack-screen { overflow-x: hidden } was applied to the same div as .jp-admin-page (via <AdminPage className={ … }>). Browsers promote the unset overflow-y: visible to auto when the other axis is hidden/scroll/clip, so .jp-admin-page was silently turning into a scroll container. Once the inner middle reached its scroll end, wheel events propagated to .jp-admin-page and shifted the whole page (header + middle + footer) up under the fixed #wpbody-content, exposing blank space below the footer. The rule pre-dated the recent 100vw breakout cleanup and was no longer load-bearing — the mixin's inner middle already handles horizontal overflow with overflow: auto, and #wpbody-content is the ultimate clip boundary.
• @automattic/jetpack-base-styles/admin-page-layout mixin: add overflow: visible on .jp-admin-page. Defensive, so any future consumer that drops a className with overflow-x: hidden on the AdminPage root won't reintroduce the same bug. Mixin specificity (body.jetpack_page_<slug> .jp-admin-page) beats consumer single-class selectors, so the override wins.

Related product discussion/links

• Stacks behaviorally on My Jetpack: adopt jetpack-admin-page-layout mixin on the main page #48503 (My Jetpack mixin adoption) and My Jetpack: fix onboarding interstitial squeezed by admin-page-layout mixin #48536 (.jetpack-admin-full-screen opt-out fix).
• Forward-compatible with Update Admin UI package and adapt to use visual prop for Jetpack logo #48410 (admin-ui 2.0): that PR renames .admin-ui-page* → .jp-admin-page__page* but does not touch .jp-admin-page itself, so the rule added here keeps applying after that update lands.

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

No. CSS/layout-only change.

Testing instructions

Repro the bug on trunk:

1. Visit admin.php?page=my-jetpack#/overview.
2. Scroll the inner middle (mouse wheel inside the page content) until it reaches its end (past the stat cards / product grid).
3. Continue scrolling down.
4. Bug: the entire page (Jetpack header + content + JetpackFooter) shifts up together, revealing a blank white area below the footer.

With this PR applied:

1. Same steps. Scroll past the inner middle's end.
2. Expected: scroll just stops. The Jetpack header stays pinned at the top, the JetpackFooter stays pinned at the viewport bottom. No blank space.

Smoke-test on the other migrated mixin pages — the layout should be unchanged. .jp-admin-page should be overflow: visible (not a scroll container) on each:

• My Jetpack (?page=my-jetpack) — Overview, Products, Help tabs
• Protect (?page=jetpack-protect)
• Social (?page=jetpack-social)
• VideoPress (?page=jetpack-videopress)
• Backup (?page=jetpack-backup)
• Newsletter (?page=jetpack-newsletter)
• Search (?page=jetpack-search)
• Boost (?page=jetpack-boost)
• Activity Log (?page=jetpack-activity-log)

A quick console probe at each: getComputedStyle(document.querySelector('.jp-admin-page')).overflow should report visible, and only the inner .container--SqdhU.fluid--OZC_9 (or in PR #48410's structure, the .jp-admin-page__page > :not(header):not(.jetpack-footer) middle) should report auto.

[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 or WordPress.com Site Helper), and enable the fix/my-jetpack-admin-page-overflow-scroll branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/my-jetpack-admin-page-overflow-scroll

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/my-jetpack-admin-page-overflow-scroll

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!

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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. 🤷

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