I have the same issue - I Drag and drop a scenery photo in first layer - then a portrait on green screen in next layer
When the second layer appears, I see the scenery layer in the layers window, downsize and index to the upper left.
If I use the chroma Key at this point - It puts the scenery in the top left and off sets the portrait
Tried your zoom suggestion as follows
I have deselected top layer and tried to zoom scenery to fit the canvas - but when I reselect the top layer it is zoomed larger as well
Any suggestions