Summary

Item dividers in SelectPanel (and FilteredActionList) are invisible even when showItemDividers={true} is set. The divider pseudo-elements exist in the DOM with correct styles but are visually clipped.

Root Cause

The content-visibility: auto performance optimization in FilteredActionList.module.css implicitly applies contain: paint (MDN reference), which clips any content that overflows an element's box — similar to overflow: hidden.

Item dividers are rendered as ::before pseudo-elements on ActionListSubContent, positioned at top: -7px (absolutely positioned relative to the sub-content span). This places the 1px divider line just above the <li> element's boundary. With paint containment active, this overflow is clipped and the dividers become invisible.

The existing code already had a comment acknowledging this same issue for the active indicator ::after pseudo-element (which is why focused/active items are excluded from the content-visibility: auto rule), but dividers were not accounted for.

Fix

Added a CSS rule that resets content-visibility to visible for items inside lists with data-dividers="true":

& [data-dividers='true'] .ActionListItem {
  content-visibility: visible;
}

This disables paint containment only when dividers are enabled, preserving the rendering performance optimization for all other lists.

Verification

Tested in Storybook at Components / SelectPanel / Features / With Item Dividers — dividers now render correctly between items.