Decided with Bryan on 2026-09-18: a child's frame is forwarded through
spans, stacks and scrolls and narrowed only by what is decided from above,
so rel is never a fraction of a self-sized box; placements are parts of
the parent's extent in frame units from its start, so a moved extent
re-places its children exactly; the placement pin becomes a symbolic
length; and a local redraw asks at the offer placement without deferring.
Lists the steps in order with the check for each and the failures that
mean stop rather than patch.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
iris-aiandClaude Fable 5.1
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