I started having a similar problem when I purchased this new Microsoft Mobile Mouse 3500 but it does not zoom even when outside the canvas.
The cheap $10 Gear Head Model MP2125BLU that I bought works fine for CTRL-SCROLL zooming, but has lots of other double click issues and eats batteries.
The Dell Wired Mouse that I have works perfect and Zooms as desired.
All 3 mice are using the same standard Windows 7 driver, I think, I never installed a driver, so they may pick different ones automagically.
The moral of this story is don't buy cheap mice.
I did it twice in the last 2 weeks, not I plan to go back and get the $39 version, which my son bought and it works perfect.
Tommie
