"But you can also click-and-drag the mouse pointer to create a zoom rectangle. Then, the action of this rectangle is better understood if the "Allow window resizing" option is unchecked"See if I can say this in Midwest speak ;-)
When the "Auto-resize window
is checked and you click on the image then the whole image is zoomed or magnified.
When the "Auto-resize window"
is not check and you click and drag your cursor you will create a box and that area only is magnified and fills the window box that you started with.
I learned something today, it is a good day then!