If the model is too large to fit into the main view, the latter displays only a part of it. You can pan the diagram to bring those parts of the model you want to focus on into the viewport.
The main view
A scrollbar slider will appear only if needed.
Move the diagram’s viewport by dragging the blue rectangle in the Overview view. The Overview is marked with an orange rectangle in figure "The itemis Model Viewer workbench". You will find details on using the Overview in section "The Overview view" .