[VKS]

var currentCell;

grid.on('mouseover', function(e,t) {
	var rowIndex = grid.getView().findRowIndex(e.target);
	var cellIndex = grid.getView().findCellIndex(e.target);
	var cell = grid.getView().getCell(rowIndex, colIndex);
	
	if (currentCell !== cell)
	{
		currentCell = cell;
		
		closeTooltip();
		showTooltip(currentCell);
	}
});

Упрощенно как то так...