ConstDeclared on both the header strip and the cells, which a SheetLayers does:
<WorksheetElement sheet={sheet}>
<SheetLayers surface={['rowHeaders', 'cells']}>
<CellInsertLayer />
</SheetLayers>
</WorksheetElement>
The button is offered from the headers only: hovering the cells shows nothing. Which cells an insert
covers, and whether there is one at all, are the host's, through resolve. Left alone it inserts
entire rows (columns) wherever the sheet's protection allows.
An insert button in the headers, on the border nearest the pointer, as Word shows one in the margin beside a table. Pressing it inserts cells at that border; while it shows, a line across the cells previews where.