Hi Alan,
Once the margins are changed, I have to click the image, then hold the right mouse button for a few seconds, then fit to page again.
You must have had Freehand mode set for this to happen. With it off, the image will automatically centralise when the margins are changed.
The appearance of the red "F" top right of the preview page changes for Freehand OFF and ON. The background of the F goes lighter for OFF.
See screenshots below.
Regarding
In Photoshop, I have added an inset thin line that is exactly 1" from each edge of the paper on all four sides.
You can do that easily in QU as a print feature using Photo Mat borders. It saves modifying and saving a new copy of the image and is extremely flexible in the way you can set borders, their dimensions, colour and number.
See learning video
https://www.youtube.com/watch?v=Rl8Q4fP7rYM&feature=youtu.be&hd=1and 3rd screen shot below.
Terry