[wieslawsoltes]

Problem and result

Unchanged Code OSS uses .monaco-list-row:has(+ .monaco-list-row.selected) and p:has(+ [data-code] > .chat-codeblock-pill-widget). WebScene parsed sibling-leading relative selectors and compiled reverse invalidation routes, but native matching rejected every + and ~ arm.

Anchor each relative arm at :scope, search only its reachable descendant or following-sibling region, and reuse the ordinary compiled selector matcher for the full chain. Class and attribute changes use the existing reverse routes; child-list changes revisit the surviving sibling subjects so final-sibling removal cannot retain stale style.

This is the next focused slice of #237 after #420. It does not touch FileReader/task scheduling, iframe/Window messaging, cache/frame resource paths, AppScene, vscode-demo, or consolidation #76.

Validation

• Chrome 153: 7/7.
• Native pre-fix control: 1/7; candidate: 7/7.
• Servo selector/parser and invalidation-plan executable: pass.
• Adjacent nested-functional, attribute-invalidation, and structural-scaling native filters: pass.
• 128 affected sibling groups × 1,000 paired transitions: identical seven-counter results with 32 and 1,024 unrelated rules/nodes; zero fallback visits; 3,097/3,356 ms; byte-exact settled V8 heap; bounded native nodes and RSS.

Detailed evidence: docs/validation/css-relative-sibling-has-20260918.md.

Refs #237, #235, and #227.