I’ve seen this one on this thread, but the OP disappeared after the request so it was closed.
Basically, at this moment the “add line” button appears on any point between a block, although of course you can only place a new line above or below it.
I think that it feels a bit “chaotic”, also considering that the button doesn’t move “smoothly” even on a 144hz monitor. So the easiest solution would be showing just 2 positions for it.