tableViewSelectionDidChange with addSubview and removeFromSuperview in table row - swift

Question

I looked for posts for issues with all 3 functions listed in the title, but my problem is not discussed anywhere. Frankly, my problem is such that it defies logic IMHO.
On selection of a row in my view-based NSTableView, I want to show a "Save" button in my last (aka "Action") column. So on viewDidLoad() I created my orphan button. Then on tableViewSelectionDidChange() I remove it from the superview and add it to my column's NSTableCellView. When there's no row selected, there is no button to show. Simple. Below code works perfectly.
func tableViewSelectionDidChange(_ notification: Notification) {
let selectedRow = tableView.selectedRow
btnSave!.removeFromSuperview()
if selectedRow > -1,
let rowView = rowView(selectedRow),
let actionView = rowView.view(atColumn: Column.action.hashValue) as? NSTableCellView {
actionView.addSubview(btnSave!)
}
}
Wait. Did I say Perfect? It works as long as I use mouse to change the selected row, including selecting below the last row in table which removes selection and any button in previous selected row. However, when I use the keyboard Up/Down keys to change the selection, it does not work.
First I thought the function will get called only if I use mouse to change row selection. However, that's not true. That's true for tableViewSelectionIsChanging as per docs and as per facts. But for tableViewSelectionDidChange docs don't say it will only work when mouse is used and the facts bear that out. I put print statements and function does get called. I stepped through debugger as well. The mind boggling part is - the method to remove button from superview works, but the one to add button as subview does not work - and only if I use keyboard. How is it possible that the same exact code executes but I get two different outcomes?
Adding remaining functions
I use this to get selected row
private func rowView(_ rowIndex: Int, _ make: Bool = false) -> NSTableRowView? {
return tableView.rowView(atRow: rowIndex, makeIfNecessary: make)
}
I call this in viewDidLoad to create my orphan button
private func createButton() {
btnSave = NSButton(frame: NSRect(x: 10, y: 0, width: 22, height: 16))
btnSave?.title = "S"
btnSave?.setButtonType(.momentaryPushIn)
btnSave?.bezelStyle = .roundRect
}