Admin page layout: clip Boot's stage so it can't add a second scrollbar

[dhasilva]

Fixes #

Proposed changes

Fixes a second scrollbar inside the frame on the Search dashboard's Overview tab when it's served through wp-build. Needed before the flag is retired in #52506.

• jetpack-admin-page-layout-wp-build now clips Boot's stage (overflow: clip). It only applies to stages that contain .jp-admin-page, and it matches both stage class shapes (boot-layout__stage and the hashed __stage) the same way the existing flex guard does.
• Search's route now imports its route.scss. wp-build only bundles styles that the entry imports, and Search's stage.tsx never imported route.scss. So the wp-build mixin was never applied to Search at all. It imported the legacy entry's admin-layout.scss instead, which only has the base mixin. route.scss is a superset of it. Search renders no Breadcrumbs nav, so the clip is the only rule that changes anything.

Why the stage scrolled

The layout mixin scrolls the page's middle, and says #wpbody-content is "the ultimate overflow: hidden boundary". wp-build puts Boot's stage inside that boundary with overflow-y: auto. The middle has no position, so an absolutely positioned element in it anchors to .jp-admin-page__page and escapes the middle's clipping. With the old layout, #wpbody-content clipped these elements. Under wp-build, they overflow the stage instead.

On Search, two screen-reader elements at the bottom of the Overview tab do this: the record meter's "More info" <span> and RecordMeterBar's summary <table>. A <table> ignores height: 1px, so the "hidden" table is still 123px tall. The stage overflowed by exactly its bottom edge: 299px on a 988px-tall viewport.

Why not the other fixes

• position: relative on the middle also removes the stage scrollbar. But it moves Boost's absolutely positioned "Your site got faster" card 80px down, since the card is anchored to .jp-admin-page__page. It also gives Search 42px of blank scroll from the table.
• hidden instead of clip: a hidden stage is still a scroll container, so focusing clipped content could scroll it and shift the whole page.
• A Search-only rule would leave the same trap for the next dashboard with a hidden element below the fold.

Effect on other dashboards

None. Measured live: My Jetpack, Social, Backup, VideoPress, Newsletter, SEO, Forms and Boost all overflow their stage by 0px, so clipping it changes nothing on them. Search's wp-build dashboard is still behind the default-off search-wp-build flag on trunk, which is why neither changelog entry is user-facing.

The window scrollbar that can appear beside the frame is unrelated. It shows when the admin menu is taller than the viewport, which the mixin accepts deliberately.

Related product discussion/links

• Blocks Search: retire the wp-build flag and the legacy dashboard bundle #52506 (retire the Search wp-build flag), which also deletes the now-unused admin-layout.scss
• Follow-up to Search: serve the dashboard through wp-build, UI unchanged #52416 (JETPACK-2568)

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

No.

Testing instructions

• Build: jp build packages/search.

• Enable the flag: wp companion feature-flag enable search-wp-build, or add_filter( 'jetpack_feature_flag_enabled_search-wp-build', '__return_true' );.

• Size the browser window so the Overview tab's content is taller than the frame, then go to Jetpack → Search.

  • Before: two vertical scrollbars side by side at the right edge of the frame.
  • After: one. It scrolls the content between the header and the footer, and the last section ("Your search index") is reachable at the bottom.

• In the console, this lists every scrolling element and should print only one:

  [ ...document.querySelectorAll( '*' ) ].filter( el => /auto|scroll/.test( getComputedStyle( el ).overflowY ) && el.scrollHeight > el.clientHeight + 1 )

• Repeat on the Settings and AI Answers tabs.

• Optional smoke test: My Jetpack, Social and Backup render and scroll as before.

🤖 Generated with Claude Code

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• 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/admin-page-layout-stage-scrollbar branch.
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