Copilot's fifth pass; both correct.
**Inline group rows are back in `frameCost`.** Omitting them was the same
failure I had just fixed for the header block: they take real table height,
so any viewport containing one asked for more data rows than `#grid` could
show, and the DOM takes its own scroll position while the client's cursor
offsets come from the larger window. Copilot's own suggestion answers the
oscillation objection — the reservation is a **high-water mark**, which
cannot oscillate because it only rises, and it rises in the safe direction:
over-reserving leaves blank space, under-reserving is the bug. It resets on
a view change so one dense document does not make every later view
pessimistic.
**`same_prefix` is replaced by `AxisFrame::shares_ancestry`**, the bulk
form, for the same reason `header_labels` and `groups` are bulk: called per
(column, level) it scanned the frame twice apiece, quadratic in the columns
drawn and growing with the horizontal offset, where the pre-anr9 widget's
materialized lookup was O(1).
`same_prefix` is **deleted** rather than kept beside it — two answers to one
question is the thing this PR exists to remove, and keeping a per-position
form that nothing calls would be an invitation. `shared_ancestry_widens_as_
the_level_rises` checks both arms, because a version comparing whole tuples
passes the leaf arm and a version comparing only the outermost label passes
the outer one.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Edward Langley
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