I edit 2d drawings in model space through paper space viewports. I'll be working on the model through a viewport, and suddenly I'm not in model space anymore. The viewport has closed and I'm adding linework and dimensions in paper space. Once I discover this, of course, I have to erase the work in paper space open the viewport and do all that work again. I cannot figure out what is causing the open viewport to close. This is a new problem, occurring in the last month or two. Any ideas?