Working for weeks on acad 2018 mechanical dwg that has 2d and 3. On Friday, XYZ coordinate, while in modelspace (not in a viewport) went askew, with z coordinate slightly off normal axis. My nav cube also askew. I set UCS to world, the view zooms out and rights itself, showing all objects. But when I zoom in next, I often get blank screen: it zooms too far and I have extreme difficulty zooming/scrolling and I notice that nav cube is askew and so is my model. Also, perspective of 3 d objects is different than I've seen in 3d: displays as if parallel lines have vanishing points. Did I fat finger some setting? It's almost too hard too work in modelspace. UCS world and XYZ axis is fine when going to MS via viewports.