Place the rulers' origin at the top-left corner of the canvas
(screen space) bounding box, and set their scale to the image-
space scale along the screen-space horizontal/vertical directions
(in other words, measuring a distance using the rulers should
give the same results as the measure tool; note that rotation
comes into play here only when the horizontal and vertical
image or screen resolutions are different, since otherwise the
scale is direction invariant.)

Make scrollbar step match ruler step under the new behavior.