If you want the Viewer to show overscan, that is any pixels extending beyond the left/right or top/bottom of the frame, you can check the show overscan box in the Viewer settings, press Alt+O, or check the option in the Viewer right-click menu. Viewing pixels beyond the edges of the frame can be useful if some of your nodes need to have access outside the frame. You can also control the amount of overscan shown by specifying the pixel amount in the overscan field.