feat(VirtualSelectList): add virtualized list for Select/Menu dropdowns

[stefanonardo]

Issue #44

Summary

• Add VirtualSelectList, a virtualized replacement for PatternFly's SelectList that renders only visible items in a scrollable dropdown, enabling Select menus with thousands of options without DOM performance degradation
• Add innerScrollContainerClassName prop to VirtualGrid so non-table consumers can override the hardcoded pf-v6-c-table__tbody class
• Add documentation examples (basic, typeahead with filtering, multi-select) and 14 unit tests

Motivation

In OpenShift Console PR #16252, the Search page Resources dropdown has 1000+ items. After algorithmic optimizations (O(n²) → O(1) lookups, memoization), the remaining bottleneck is PatternFly's Select component measuring DOM geometry (refCallback forced reflow) after inserting all items. Even capped at 100 items, INP is 270ms ("needs improvement" on Core Web Vitals).

VirtualSelectList reduces INP from 270ms → 107ms ("good"), a 60% further reduction by rendering only ~10-20 visible items instead of 100.

Metric	Before (100-item cap)	After (VirtualSelectList)
INP	270ms (needs improvement)	107ms (good)
Processing duration	~200ms	77ms
Presentation delay	~70ms	22ms

Usage

Drop-in replacement for <SelectList> inside a composable <Select>:

import { Select, SelectOption, MenuToggle } from '@patternfly/react-core';
import { VirtualSelectList } from '@patternfly/react-virtualized-extension';

<Select isOpen={isOpen} onSelect={onSelect} toggle={toggle} isScrollable>
  <VirtualSelectList
    rowCount={items.length}
    rowHeight={36}
    maxHeight={300}
    rowRenderer={({ index, style, key }) => (
      <SelectOption key={key} style={style} value={items[index]}>
        {items[index]}
      </SelectOption>
    )}
  />
</Select>

Design decisions

• Wraps VirtualGrid with 1 column, N rows — mirrors VirtualTableBody pattern
• Renders <ul role="listbox"> as outer container via scrollContainerComponent
• rowRenderer callback (not children) — required for lazy virtualized rendering, matches VirtualTableBody.rowRenderer naming
• Internal keyboard navigation — ArrowUp/Down/Home/End with scrollToRow
• Functional component — modern React style (vs VirtualTableBody's class component)

Known limitations

• Browser Ctrl+F won't find off-screen items — typeahead filter covers this use case
• Items are absolutely positioned, so the dropdown menu won't auto-size to content width — consumers should set popperProps={{ minWidth: '...' }} on the <Select>
• No grouped/categorized support — flat lists only; <SelectGroup> support is a follow-up

Test plan

• yarn build passes (ESM + CJS)
• yarn test — 26 tests pass (14 new for VirtualSelectList, 12 existing unchanged)
• Tested in OpenShift Console against a live cluster with 1000+ API resources
• INP profiled at 107ms (good) via Chrome DevTools Performance trace