Zachary D. Rowitsch 20e241bd03 Implement margin collapsing through empty elements (CSS 2.1 §8.3.1) ...

Self-collapsing elements (zero height, no padding/border, no BFC, no line
boxes) now have their top and bottom margins collapse together, and child
margins propagate through parents with auto height and no bottom
padding/border. This replaces the pairwise collapse_margins() function
with a MarginSet accumulator that tracks max positive and min negative
margins separately, correctly resolving chains of 4+ margins.

Key changes:
- Add is_self_collapsing() post-layout predicate and gather_all_margins()
- Add gather_margin_bottom() for bottom margin propagation with a guard
  against propagation when max-height constrains the parent
- Replace prev_margin_bottom with MarginSet-based pending_margins in the
  sibling layout loop
- Handle clearance + self-collapsing interaction correctly

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

2026-03-01 01:04:53 -05:00

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