[nstepien]

Calculated columns have a new getColSpan(args) method, which returns the number of columns that a cell spans: the value returned by the column's colSpan, clamped, or 1. Cells, keyboard navigation, the editor, and the drag handle all use it, so they agree on each cell's span.

Spans are clamped instead of ignored. A span is clamped to the start-frozen, unfrozen, or end-frozen columns, whichever the column is part of:

• A span that crossed the edge of the frozen columns used to be ignored, and the cell spanned one column. The cell now spans as many columns as it can.
• A span could also go past the last column, if the column was end-frozen, or unfrozen without end-frozen columns. It's now clamped to the last column.

Drag handle fixes:

• The drag handle called colSpan directly. When the cells ignored a span, the drag handle still spanned the columns that the span would have covered, and showed up at the end of the last one.
• When scrolling, the drag handle of a start-frozen cell sticks at the end of the cell's first column, so it moved to the middle of cells that span columns. It now sticks at the end of the last column that the cell spans.

API changes:

• CalculatedColumn has a new getColSpan member.
• colSpan in RenderCellProps and in the callback of mapViewportColumns is now always a number: 1 instead of undefined for cells that don't span.
• Cells that don't span still don't render aria-colspan.
• ColumnFrozen is now false | 'start' | 'end'. Column.frozen still accepts true, as an alias for 'start', and calculated columns normalize it to 'start'.

Tests:

• New tests in colSpan.test.ts check that spans are clamped to each group of columns in the header row, rows, and summary rows, and to the last column.
• Two tests in dragFill.test.tsx check that the drag handle straddles the end of a cell whose span is clamped, and the end of a start-frozen cell that spans columns, before and after scrolling.
• These 4 tests fail on main.

The drag handle fix was split from #4005.

vp run ci passes.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (81f2df3) to head (2ba85db).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4251      +/-   ##
==========================================
+ Coverage   99.13%   99.22%   +0.09%     
==========================================
  Files          46       44       -2     
  Lines        2207     2207              
  Branches      977      971       -6     
==========================================
+ Hits         2188     2190       +2     
+ Misses         19       17       -2     

Files with missing lines	Coverage Δ
src/Cell.tsx	100.00% <100.00%> (ø)
src/DataGrid.tsx	99.26% <100.00%> (+0.18%)	⬆️
src/EditCell.tsx	98.98% <100.00%> (+0.04%)	⬆️
src/GroupCell.tsx	100.00% <100.00%> (ø)
src/HeaderCell.tsx	98.29% <100.00%> (ø)
src/SummaryCell.tsx	100.00% <100.00%> (ø)
src/hooks/useCalculatedColumns.ts	100.00% <100.00%> (ø)
src/hooks/useViewportColumns.ts	100.00% <100.00%> (ø)
src/utils/activePositionUtils.ts	100.00% <100.00%> (ø)
src/utils/index.ts	90.90% <ø> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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