Administration: Fix responsive list table toggle-row button positioning

[itzmekhokan]

[62838] / #32892 made the primary column a <th scope="row"> and the checkbox column a <td>, but two selectors in the @media screen and (max-width: 782px) block of wp-admin/css/list-tables.css still only match td.

As a result the primary cell never gets position: relative, so the absolutely positioned .toggle-row button resolves against #wpbody and every row's button stacks on the Help and Screen Options tabs (measured top: 56, left: 734 on edit.php at 782px). Clicking Help expands a row instead. On expand, the primary th also stays table-cell while the sibling tds collapse to 8px blocks.

This adds a th selector to both rules, scoped to tbody so thead/tfoot are untouched, and excludes .check-column from the .is-expanded rule so the checkbox cell stays table-cell.

To test: at a viewport of 782px or narrower, open Posts, Pages, Comments, Users or Media (list mode). Each row should have its own chevron at the right edge of the title cell, the Help and Screen Options tabs should work, and expanding a row should stack its columns full width with their labels.

Trac ticket: https://core.trac.wordpress.org/ticket/65763

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Reproducing the bug at the 782px breakpoint, tracing it to [62838], and drafting the CSS change. I reviewed the diff, directed the scoping decisions, and verified the behaviour in the browser across the affected screens.

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Pull request overview

This PR fixes responsive (≤782px) list table row-toggle (“chevron”) behavior after the primary column switched from td to th scope="row", ensuring the toggle button positions within the correct cell and expanded rows stack correctly.

Changes:

• Extend the responsive positioning-context rule to also match tbody th:not(.check-column) so .toggle-row’s absolute positioning resolves per-row.
• Extend the responsive expanded-row stacking rule to include th and exclude .check-column, keeping the checkbox cell as table-cell while other cells become blocks.

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[github-actions]

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