The image viewport displays, the Output Image, the Reference Image, and all the gizmos and images used by the Active node. In this particular image, the Active node is the "Resize" Node: node's background is green with red border in the Node Graph Window. The Image window shows the image, but also the Resize' gizmo in light green bounding the picture, and in light yellow the gizmo of the original picture before being "Reframed" and "Resized".
If the Node Graph is being displaced all mouse input is sent to the Nodes Graph to allow the user to add new nodes, connect nodes and move nodes. Some nodes can display gizmos and images in the Viewport, and can also process mouse actions.
Alt++LMB click and drag on the viewport to pan the view in X and Y.