Merged Cell re-implementation

There are several known issues/limitations with the current implementation of merged cells:

• Thick border problem on merged cells

• If formatted gets white border.

• Problem with merged cell when scrolling

• Merged cells and freeze pane

• Merged cells, that are bigger than one Row and are combined whith hiderow, are changing there shape.

• Copy/fill and merged cells

• Moving cell reference onto merged cell

• Format painter doesn't copy merge info

• Merged cell is unmerged when shifting cells up in deletion

• Showpict in merged cell cut off when exported to pdf

• A merged cell causes "freeze" in Worksheet when moving down with shift key+down arrow. These issues cannot be easily fixed, i.e. require re-implementation of the merged cells subsystem which per se is a bigger effort. #16557