Issues coupling NSDraggingSource w/NSMatrix

Question

Picture this sparsely populated NSMatrix, five coloured columns (0-4), with the most recently added column in orange. These are populated with stand-in objects (random numbers) with the (x,y) coordinates noted. "Blank" cells have stand-in NULL objects. This is kinda working.

I have four data "sources" (A-D) at the top that I want draggable onto the matrix. They are tagged with numbers that represent target rows (0-3). I can't get these to drag even though I think I've done NSDragSource correctly.

Example behaviours:

If I drag cell B (with 0,2) onto the "centre" area of the column (4,y), its contents will merge with the two empty cells (rows 0 and 2). If I drag onto the left or right area of the column, I will get a prepended or appended column -- in this case, a sparse one.

If I drag cell A (with 0,1,2,3) onto any cell, it will create a new column.

If I drag cell D onto column (0,y) or (4,y) it will merge; dragging it anywhere else will create new sparse column. Doing so with C anywhere will create a new sparse column.

As one could surmise, this complex merging behaviour has led me here rather than, say, to NSTableView.

I don't so much have a problem of identifying the left/center/right areas or merging the data or creating new columns -- I need help creating the proper NSPasteboard interactions and/or the NSDragging setup: (I ultimately want to using a custom NSManagedObject in Core Data that will be identify which rows are populated, but I'm starting here).

1. I'm trying to prototype a NSStringPboardType attached to the four NSTextFields that will be draggable. No dice.
2. I can make my NSMatrix receive strings (or an NSFilenamesPboardType) from the pasteboard. No problem there.

Once I receive the appropriate incoming cell (string), I can riffle through it and figure out whether to merge with the current column, or create a new column -- again, that's not the issue.

Things I have done...

1) Subclassed the NSTextField...

@interface CellStringView : NSTextField <NSDraggingSource>

2) Registered the text fields' dragged types & attempted a drag session...

- (void)awakeFromNib {
    [self registerForDraggedTypes:@[NSStringPboardType]];
}

- (NSDragOperation)draggingSession:(NSDraggingSession *)session
sourceOperationMaskForDraggingContext:(NSDraggingContext)context {
    if (context == NSDraggingContextWithinApplication) {
        return NSDragOperationCopy;
    }
    else
        return NSDragOperationNone;
}

-(void)dealloc {
    [self unregisterDraggedTypes];
}

My draggingSession:sourceOperationMaskForDraggingContext: method is never even called.

I'm new to this NSPasteboard stuff, so any help would be appreciated.