[nstepien]

Applies What's up with monomorphism? and Elements kinds in V8 to the grid. Only the first one called for changes.

Cause

useCalculatedColumns created each calculated column with { ...rawColumn, … }, so the column's hidden class (shape) depended on which optional properties the raw column set, and in what order. With the columns of the CommonFeatures demo, the 16 calculated columns had 9 different shapes. V8's inline caches handle up to 4 shapes per property access, so the column.* reads that run for every rendered cell, in Cell, getColSpan, getCellStyle, getCellClassname, isCellEditableUtil, and others, went megamorphic.

Changes

• Calculated columns set every property in a fixed order, undefined when unset. To match, CalculatedColumn redeclares the optional Column properties as required, with Column's own property types, like Column<TRow, TSummaryRow>['cellClass'], so leaving one out is a type error. Unlike Required<Column>, this keeps undefined in their types whether or not apps enable exactOptionalPropertyTypes.
• setPosition builds the active position from idx and rowIdx instead of spreading the position it receives, and the five position literals written as { rowIdx, idx } now use { idx, rowIdx } like the others.
• AGENTS.md documents the convention.

Measurements

Measured on the packed lib/ with Node 26 (V8 14.6), using the CommonFeatures columns and enableVirtualization={false}:

	Before	After
Shapes among the 16 calculated columns (%HaveSameMap)	9	1
Load sites that read calculated column properties (--log-ic)	34 megamorphic, 2 polymorphic, and 3 monomorphic	all 39 monomorphic
renderToString of 300 rows × 16 columns, median	11.04 ms	10.74 ms

The render time improves by 2.7%, across 8 alternating pairs of processes. React's own work dominates rendering, and DOM work in the browser dilutes the gain further, so this is a small, steady improvement rather than a big one.

Reads of the raw columns stay megamorphic, since users create them, but they only run in useMemo when columns changes.

Behavior and type changes

• Calculated columns now have every Column property as an own property, so Object.keys(column) and 'cellClass' in column return different results than before.
• Properties that aren't part of Column are no longer copied to calculated columns, as they aren't supported.
• Every Column property is required on CalculatedColumn, so code that creates CalculatedColumn objects itself, like test mocks, has to set all of them.

Elements kinds

No changes needed. The array patterns the grid uses all create packed arrays on V8 14.6, and no hot loop reads past the end of an array.

Auditing the out-of-bounds reads found a crash, which #4223 fixes.

Verification

• A new test checks that all calculated columns have the same properties in the same order. It fails before this change.
• vp run ci passes.

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (e36fd41) to head (3bcf373).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4222   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files          45       45           
  Lines        1956     1957    +1     
  Branches      728      728           
=======================================
+ Hits         1928     1929    +1     
  Misses         28       28           

Files with missing lines	Coverage Δ
src/Cell.tsx	100.00% <100.00%> (ø)
src/DataGrid.tsx	98.88% <100.00%> (+<0.01%)	⬆️
src/GroupedRow.tsx	100.00% <100.00%> (ø)
src/SummaryCell.tsx	100.00% <100.00%> (ø)
src/hooks/useCalculatedColumns.ts	100.00% <100.00%> (ø)

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