Thank you very much @Okavango for the diagram.
I will carefully consider each suggestion. One thing is already certain is that the snap zone will be implemented in the ortho mode. But I fear that it should be calculated on the 2d variables (x and y coordinates of the screen). If so, I shall make a very thorough study about the view_matrix. And that will slow down the development.
The work will be for next week (I need to sort some things out for now). So I will do everything more carefully. Thanks!