MouseDown events return Y-value offset by 1 pixel
For some reason, the various MouseDown events all appear to subtract 1 pixel from the Y value, while leaving the X alone. The MouseMove event, in contrast, does not adjust the Y value.
I can find no comments thus far indicating the reasoning, and the net effect can be observed by attempting to draw a horizontal line with the Shape Editing tool or the Measuring tool - after clicking the first point, draw what appears to be a horizontal line, and as soon as you click the second point, you can see the resulting line is no longer horizontal.
I will consider removing the adjustment and watching for side-effects.
1-pixel Y-offsets have been removed from all MouseDown events.