I think you have a bug with your collision/edge detection, as I somehow managed to get the line stuck. i.e. I can not move any of the blue nodes.
I get the 3D aspect of it, but I couldn't move any of the blue nodes in any direction.
The difference is that normally the game stops your line earlier. i.e. like this:
meaning you can still move at least one of the nodes.
But somehow I managed to get the line like this. Although I am struggling to reproduce it.
If I had to guess (without looking at the code), on a move, if your line would end up in an invalid location, it would move the line back to where it was. That's sensible if the current/previous line position is sensible/legal. Unfortunately if the line glitches into an illegal position, you can never move it out of that position (as to do so would mean you would have to transition through various other illegal positions, when that happens, the line would be moved to the current position).
I understand. As you said, this is a bug. And I was also able to reproduce the following steps.
1. Move the blue line to the lower side of one of the green lines, making the intersection of the green lines impassable.
2. Create the midpoint of the blue line and move this to the position of the intersection of the green lines.
3. Move the blue line so that it is in a straight line and delete the midpoint.