On the CNN homepage ~19% of paint items were exact duplicates: story-card
headlines painted 2x and badges up to 6x at identical coordinates. Root cause
was a direct inline-block child of an element establishing an inline formatting
context (IFC) being handled by two independent paint paths, plus a positioned-
auto descendant being painted both in place and after step-6 hoisting.
Three coordinated fixes in the display-list builder, each independently
necessary (verified by toggling on the real CNN page):
1. Exclude direct inline-block children of an IFC element from
`normal_flow_children`. They are atomic inline-level boxes painted once as a
self-contained unit by `render_non_inline_descendants`; including them in
normal flow painted them again via the phased step-3/step-5 passes and walked
them a second time during step-6 collection.
2. Deduplicate the step-6 positioned-descendant list by node id. A positioned-
auto descendant could be collected by both the normal-flow loop and the
has_ifc pass (overlapping subtrees), rendering its whole subtree twice.
3. Track an `in_atomic_inline_block` flag: while painting an inline-block atom's
Full render, suppress the in-place render of positioned-auto descendants —
they are hoisted to the owning stacking context's CSS 2.1 Appendix E step-6
and painted there exactly once. The flag is reset on entering any stacking
context so hoisted/own positioned descendants still paint in place.
Result on the saved CNN page: 2394 -> 1944 paint items; the badge SolidRect,
"CNN INVESTIGATES", and the lead headline now each appear exactly once.
A previously known_fail WPT reftest (flexbox margin-left: auto with inline-block
items in a position:relative flex container) now passes and is promoted.
Regression tests: test_relative_headline_in_inline_block_painted_once and
test_positioned_auto_in_inline_block_painted_once in display_list — both fail
before the fix and pass after.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Zachary D. Rowitsch
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