AO3-7160 Fix clipped autocomplete dropdown when assigning a pinch hitter

[brianjaustin]

This reverts commit 69d6268. It reintroduces #5699.

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Issue

https://otwarchive.atlassian.net/browse/AO3-7160

Purpose

Fixes the clipped autocomplete dropdown when assigning a pinch hitter on challenge maintainer assignments.

The dropdown was being clipped by ancestor elements using overflow: hidden (fieldset and form dl in 01-core.css).

Changes made:

• Added maintainer-assignments class to the maintainer assignments fieldset.
• Added scoped CSS override (overflow: visible) for that fieldset and its dl.index to allow the dropdown to render fully without affecting global styles.

Testing Instructions

Follow the reproduction steps in Jira (AO3-7160):
https://otwarchive.atlassian.net/browse/AO3-7160

Expected result:

• The autocomplete dropdown is fully visible and not clipped.

Note: This is a visual/CSS fix and is not easily covered by existing automated tests.

Screenshots

Before:

After:

Credit

Name: Noah Bravo
Pronouns: they/them