Comment 6 for bug 410267

More information on the offset:

*) Screen mode
The amount of x- and y-offset equal the space between window border and visible canvas (image canvas + space around), like ruler/menu bar:

http://img.xrmb2.net/images/770432.png

Removing the rulers reduces the offset by the rulers widths:

http://img.xrmb2.net/images/945369.png

*) Window mode
The stylus' x- and y-position within the visible canvas area seems to be equal to the relative position of the cursor on screen, e.g.
cursor position x (on screen) = screen width * 0.75
cursor position y (on screen) = screen height * 0.5
=>
stylus position x (within canvas) = canvas width * 0.75
stylus position y (within canvas) = canvas height * 0.5:

http://img.xrmb2.net/images/171503.png