GIMP currently displays padding round an image (sometimes called
a matte, like the cardboard used between a picture frame and the
picture) when the image is smaller than the window.

It's useful when you're working near the edge of a large image
to be able to go "off the edge" without accidentally resizing
the window or scrolling or picking up a guide.

So I'd like to be able to toggle a matte of (say) 50 screen
pixels (a cm or so) with a keystroke and/or view menu option;
this matte would be like the canvas padding except it would
be shown regardless of image size.