The target notebook widget.
If only one cell is selected, indicates whether to merge it with the cell above (true) or below (false, default).
The widget mode will be preserved.
If only one cell is selected and mergeAbove is true, the above cell will be selected.
If only one cell is selected and mergeAbove is false, the below cell will be selected.
If the active cell is a code cell, its outputs will be cleared.
This action can be undone.
The final cell will have the same type as the active cell.
If the active cell is a markdown cell, it will be unrendered.
Merge the selected cells.