Maybe just try this. If you have mouse coordinates xm, ym, viewport offset vx, vy and viewport size dx, dy, you compute coordinates to pass to this screenToWorld like this:
x = (xm - vx) / dx
y = (ym - vy) / dy
and just to be clear, by mouse coordinates xm and ym I mean normalized coordinates, so if you have screen coordinates x and y, and device screen size device.clientRect.width and .height:
xm = x / device.clientRect.width
ym = y / device.clientRect.height