4. But a "Ctrl-Click" could just expand-collapse the clicked node
Optionally, the user could keep unchanged the nodes that were already on the screen and only toggle the status of the clicked node (status = collapsed or expanded)
In this mode (that would be achieved, say, by using Ctrl-Click rather than the ordinary Click), the user is allowed to stop "playing the butterfly". The trail that has led to the current map remains visible as long as the "Ctrl-Click" is used instead of the "Click".