Keep root-level :where()/:is() combined with other pseudos

[AdrianBannister]

Problem

A CSS rule whose selector is a root-level compound of pseudo-classes was purged even when the element/class it targets is present in the scanned content. Examples that were wrongly dropped:

• :where(.a):not(_) / :is(.a):not(_):not(_) — :where(.a) alone and .a:not(_) alone were both kept; only the combination broke.
• :is(h1, h2, h3), :is(body) — selector lists purged regardless of matching elements.
• :is(button, input):not(.some-class) — :is() combined with :not().
• :where(:not(.alignleft):not(.alignright):not(.alignfull)) — Gutenberg's :where()-wrapped :not() chains.
• bare :hover, :not(:hover) — pseudo-classes with no preceding selector.

These were reported in #978 and #1282.

Root cause

In shouldKeepSelector, the per-node loop starts isPresent = false and only sets it from matchable nodes (class/id/tag/attribute); an absent matchable node early-returns false, while pseudo-class nodes hit the switch default and continue. So the loop reaches its terminal return isPresent with false in exactly one situation: the compound had no matchable node at all (every node was a pseudo). Nothing was actually missing, yet the rule was dropped.

The previous isPseudoClassAtRootLevel helper was a band-aid that rescued only the single-node form of this (selector.nodes.length === 1), so any compound (:where(.a):not(_), :is(...):not(...), …) still fell through and was purged.

Fix

Fix the terminal value instead of special-casing: reaching the end of the loop means no matchable part was found missing, so return true. This makes the isPseudoClassAtRootLevel special case redundant — it's removed.

The classes inside :where()/:is() are evaluated separately as their own selector nodes, so the real keep/purge decision (and the empty-:where/:is cleanup pass) already happens there — the compound level only needs to not veto it. A rule is still purged when none of the classes/tags it depends on are present (e.g. :where(.unused):not(_) → removed, verified).

Behavioural change

The only change in matching logic is that terminal return, so behaviour differs only for root-level compounds made up entirely of pseudo-classes. Anything containing a real class/id/tag is unchanged.

Selector	Content has	Before	After
:where(.a):not(_)	.a	purged ✗	kept ✓
:is(.a):not(_):not(_)	.a	purged ✗	kept ✓
:is(h1, h2, h3)	<h2>	purged ✗	kept ✓
:is(button, input):not(.x)	<button>	purged ✗	kept ✓
:where(:not(.l):not(.r))	matching ancestor	purged ✗	kept ✓
:hover, :hover:focus	—	purged	kept (matches *:hover, already kept)
:not(.unused):not(.other)	—	purged	kept (single :not(.unused) already kept; :not inverts)
:where(.unused):not(_)	—	purged	purged ✓ (unused class → still dropped)
:where(:is(.unused))	—	purged	purged ✓ (holds through nesting)
.foo:not(.x)	.foo absent	purged	purged ✓ (unchanged)
.foo:hover, *:hover	matching	kept	kept ✓ (unchanged)

Tests

Expanded packages/purgecss/__tests__/pseudo-class.test.ts with the compound :where/:is/:not cases above, including negative cases that must still purge (unused class in :where, absent tag in :is(...):not(...), nested unused). Full purgecss suite: 147 passed, 25 suites.