The static position of an absolutely positioned inline child is the
in-flow insertion point on the line where it appears. Previously this
was reconstructed after layout by walking previous siblings for a
line-box fragment. That lookup could match a fragment from an earlier
line and collapse multiple abspos children onto the same position,
especially in white-space preserving content.
Instead, drop a zero-width static-position marker into the line box at
the insertion point when each abspos child is encountered. The marker is
carried through normal line post-processing, including float intrusion,
text-align, justification, trailing-whitespace trimming and ellipsis, so
the final static position is resolved from the line itself.
Resolve marker-only trailing lines before removing them, so they can
provide static position without contributing line height.
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