Canon PowerShot S120 Manual (en)

Second windows are preview windows that you can open additionally 
for thumbnails and image previews. You can display up to 4 second 
windows for an single image to check the focus in all parts of the image, 
Select an image, and then select [Show second 
screen] from the menu that appears when you right-
click with the mouse.
A second window is displayed.
To display an additional second window, repeat the operation.
You can change the size of second windows as desired.
Click the displayed image to switch between actual pixel size 
(100% view) and fit-to-window view.
When you magnify an image in the second window, the right-
clicked position on the image will be the center for the 
Checking an Image in Second Windows