Please refer to the Wiki Documentation for the complete Mouse Action reference.
In the main SimpleIndex window you will see a sidebar on the right side, and in the middle of that sidebar are buttons that control the Mouse Action. By default the Mouse Action is set to Zoom In on the image so when you need to magnify something on an image you can quickly and easily do that.
If you clicked and dragged your mouse accidentally over the image you zoomed in on it, so to return the image to full size click on the View menu at the top of the window, then click on Full Page. The keyboard shortcut to do this is also listed beside the menu command, in this case it is Ctrl + F.
