Project files used in the tutorial: https://www.youtube.com/watch?v=TnL0i47QhqI
This project was the demo scene I made a long time ago when I was thinking about making tutorials about state machines and NPC AI.
But I was playing around with some prototype rts-style game and had quite a bit of difficulty making the selection work.
I was trying all sorts of weird things with shapecasting or trying to figure out how to create a 3D shape where the mouse is drawing its area. And while I did come up with something that worked, the moment that I used a orthographic view, or rotated the camera, things broke.
So I went back to basics, and this tutorial is the solution that I found works quite well.
Anyways, the project files are in the attachments of this post.
The scene of the tutorial is at:
tutorials/1 mouse_selection.tscn