Zachary D. Rowitsch 53dd6646b2 Implement stacking contexts and z-index per CSS 2.1 §9.9.1 / Appendix E ...

Root element now creates a stacking context unconditionally. Refactored
display list builder to correctly bucket stacking participants into three
groups (negative/zero/positive z-index) per Appendix E steps 2/6/7.
Fixed z-index:0 elements painting at step 7 instead of step 6, and
descendant stacking contexts being trapped inside positioned z-index:auto
ancestors instead of participating in the parent stacking context.

Added StackingBuckets struct, unified step-6 rendering with tree-order
merge by node_id, and 7 unit tests for creates_stacking_context().
Added 6 golden tests (213-218) and 4 integration tests asserting
paint-order invariants programmatically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

2026-03-13 19:30:31 -04:00

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