NSTableView comes with a couple of default shortcuts in an override of its keyDown(with:) method. These aren’t mentioned anywhere, so Xcode’s documentation quick help will repeat the NSView docstring to you, saying: The receiver can interpret event itself, or pass it to the system input manager using interpretKeyEvents(_:). The default implementation simply passes this message to the next responder. [Emphasis mine]
You can make two NSScrollViews scroll in concert quite easily because every scrolled pixel is broadcasted to interested parties. In TableFlip, the main table is a NSTableView contained in a NSScrollView. You can view and hide row numbers in TableFlip; but I didn’t want to reload the whole table and mess with the table model to insert and remove the first column. Instead, I use a second table view with a single column. The upside of this approach: I can animate hiding the whole scroll view with the row numbers inside easily without affecting the main table.