These options affect the standalone viewer - the Opus viewer that opens in a separate window. Options that affect the Viewer Pane are configured on a separate page.
This lets you control what the left mouse button does when you click it in on the image. The options you can choose from the drop-down are:
Below the drop-down is the following options:
This lets you control what the left button does when you double-click it on the image; the options are mostly the same as for the Left mouse button setting except you can't choose Select clipboard region or Scroll image for double-click.
This lets you define what the middle mouse button does when you click it on the image; the options are the same as for the Left mouse button setting.
This lets you control what the mouse wheel does when you turn it over the viewer window. Note that the viewer ignores this setting when viewing some file types. For example, when viewing a text file the wheel always scrolls up and down. The options in the drop-down are:
Beneath the drop-down is the Accumulate wheel movements option, which is enabled when Advance to next image is selected. This option causes wheel movements to accumulate and several quick wheel movements can move you several places forward or back in the list of images without loading each image in between. (On mice with 'smooth' wheels, this could make it easy to accidentally skip images by turning the wheel too much).