[kianbazza]

Summary

Each positional selector filtered the whole row order for every row on every store update, so a long menu paid roughly rows² work per update. The positions are now worked out once per order change and every selector is a lookup.

 isFirstInGroup(state, rowId)
   if virtualized: return false
-  row = orderedRows.find(r => r.id === rowId)
-  siblings = orderedRows.filter(r => r.groupId === row.groupId)
-  return siblings[0].id === rowId
+  return getRowPositions(orderedRows).firstInGroupIds.has(rowId)

+getRowPositions(orderedRows)      // cached per orderedRows array
+  one walk: first/last loose row, first/last group, first/last row per group

The same change applies to isFirstRow, isLastRow, isFirstGroup, isLastGroup and isLastInGroup. orderedRows is already replaced wholesale whenever a row mounts or unmounts, so the array itself keys the cache.

Evidence

ListboxStore.test.ts › "reads the row order a bounded number of times per order change": 101 rows. It counts reads of orderedRows elements while every row's six selectors run, then runs them again after a highlight change, which notifies the store without changing the order.

                                        before    after
first sweep after an order change       61,206    101    (asserted <= 101)
second sweep after a highlight change   61,206    0      (asserted 0)

• Before: AssertionError: expected 61206 to be less than or equal to 101
  After: passes, with the flags checked for the first, last, first-in-group and last-in-group rows

positional-attrs.test.tsx passes unchanged.

Merge Danger

Door: two-way

Blast Radius: small

A wrong lookup would put data-first, data-last, data-first-group, data-last-group, data-first-in-group or data-last-in-group on the wrong row, which changes styling but not behaviour. positional-attrs.test.tsx covers these attributes through filtering.

Closes UI-641

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Project	Deployment	Actions	Updated
ui-canary	Ready	Preview	Oct 10, 2026 11:34pm UTC

[linear-code]

UI-641