Establish retained stacking contexts for CSS isolation #795

Parent epic: #235
Paint owner: #241
Performance owner: #243
Visual owner: #259
Release owner: #227

Problem

Unchanged Code OSS 645f29cc3176500b4b5762ba887cf2a7f0ffdf2c uses isolation:isolate on five retained surfaces: the pixel spinner, sessions workbench/part, Chat session container, and voice glow. WebScene main before #796 did not expose the property or establish its stacking context, so positioned descendants can escape their intended paint group and reorder above sibling workbench surfaces.

Focused implementation

• expose isolation through native/managed metadata and CSSOM;
• parse/cascade/reset/serialize auto and isolate, including global keywords and mutation/removal;
• retain one packed stacking-context bit without allocating per-node strings, maps, timers, or visual-tree nodes;
• include the bit in the existing atomic stacking-context predicate used by retained paint and hit ordering;
• publish paint only for mutation, with no layout invalidation;
• keep unchanged Code OSS untouched and add no browser/platform layer.

Authored quality and performance gates

• CSSOM initial/computed value, inline mutation/removal, invalid writes, global keywords, and cascade fallback;
• an overlapping sibling fixture proving an isolated descendant z-index cannot escape, plus mutation back to auto;
• generated scene/hit order, nested/positioned/opacity/transform/filter combinations, detach/navigation cleanup, and iframe isolation;
• 4,096 isolated nodes across repeated mutation with zero added layout, bounded scene publications, stable memory, and no sustained frame demand;
• exact spinner, sessions, Chat, and voice-glow Chromium/AppScene regions.

Acceptance status

• Focused reusable implementation merged in Implement CSS isolation stacking contexts #796 at fe2a3513.
• Browser/native CSSOM, hit, and paint-order contracts passed.
• Performance, memory, and lifecycle bounds passed.
• Exact unchanged Code OSS product regions passed.

Per the active implementation-only direction, source review and git diff --check may precede merge; unexecuted gates remain zero evidence.

Merged result — 20 September 2026

PR #796 is merged into main; current WebScene head is c9a28121e5df848d2d41a00ed09dc42857ed5e67. The authored browser/native, performance, memory/lifecycle, and unchanged-product gates remain unexecuted qualification debt.