If you drag some text from one ScintillaView and drop to a different ScintillaView in the same application, no text is inserted into the destination ScintillaView.
I was able to fix this by changing in ScintillaCocoa.mm ScintillaCocoa::PerformDragOperation():
bool moving = (operation & NSDragOperationMove) != 0;
InnerView* contentView = ContentView(); bool moving = ((operation & NSDragOperationMove) != 0) && (contentView == [info draggingSource]);
However, this doesn't account for two different ScintillaView's that point to the same document (for example, from a split view).