How to maintain right angles while editing a rotated mesh?

Question

I'm trying to move an edge loop in a mesh that's not aligned to the global axis, but is supposed to have all right angles (since it was created from a cube). Even though I move the loop along the mesh's local Y axis eventually the angles on some edges deviate from 0.000/90.000 degrees as you can see here:

This stops happening for this particular edge loop after I apply the scale translation to this object, but now I'm worried it'll happen to me after I've made a lot more modifications to the mesh and wasn't paying attention to the angles.

Is this actually due to a precision loss or something else? Is applying the scale the best solution, if so I'd like to understand why, if not, what is?

Sample file: https://ufile.io/s7vwy Blender: v2.79

Answer 1

This is happening because you're moving the loop according the its locals, which is not straight because your mesh is not straight--your mesh has its own locals (either an element that you added is making it this way, or you didn't make your mesh on the grid properly)--not according to the other points in respect to the loop. How do we know this? Because if you slide the loop G x 2, you find that this doesn't happen.

In orthographic view, you can see that there's an alignment issue of some sort at the very top (middle). There's a crease there to be specific (not easy to notice, I know). Check this by going into orthographic view from the front side (Numpad 5 for orthographic, and Numpad 1 for front) and look by the Z axis. Alternatively, look at Z-axis values of your edges.

The sign of not being straight (only noticeable at distances, generally): (Note that if the lines were straight, this should never happen.)