On certain browsers, holding the Ctrl key (Windows, Chrome) or Cmd key (Mac) down while using the mouse wheel will cause the browser window itself to zoom larger or smaller. This may result in partially hiding your screen or making the interface unreadably small.
To restore the browser view to a more comfortable zoom level, either hold the Ctrl key (Windows, Chrome) or Cmd key (Mac) and zoom back to a default view using the mouse wheel, or reset the browser zoom itself.
The browser zoom is usually controlled under the top right menu - options may vary depending on your browser choice. Reset to 100% to restore Memento Yearbook to the default view.
To learn how to zoom & pan only the canvas in the editor without affecting the entire interface, consult the article entitled Zooming and panning the canvas.