Bugfix
In the Model Editor, a diagram moves so far to the right after you have switched on the design mode that it is no longer visible on the screen. It only becomes visible again when the diagram itself is dragged to the left until it is visible on the screen again.
Testing
Start the application and navigate to Administration > Development > Model Editor. Search for a diagram of a functional object whose diagram is very small, e.g. tl.demo.graph. Select the Diagram tab there. Select a type of this module, e.g. N. Then switch on the design mode. The diagram should still be visible without having to reposition it manually.