Memento Composite offers a focus-zoom feature.
To zoom in on a particular area on your canvas, mouse-over that area and scroll down with your mouse wheel.
The canvas will zoom in focused on the area where your mouse is positioned.
To pan the canvas, hold the keyboard's space bar down and click down on the left mouse button, then move your mouse to recenter the view.
To zoom out, scroll your mouse-wheel up.
To reset the canvas back to full-screen view, double-click on the Zoom slider.
The canvas will once again be fully visible.