When painting portraits from photos, I always want to have my canvas
side-by-side with a reference image. The only way I can display a reference
image within GIMP is to un-check the single-window mode option and set up a
second canvas to hold the reference image.
There should be a way to hold a reference image while still in
single-window mode. Maybe a dockable image reference dialog with options to
zoom-in/out and panning, more like the navigation dock - but not affecting
the image on the canvas. Or add an option to zoom-in/out the image(s) in
the document history dialog which I can also use for reference images.