By accident, I today found a solution (for my very particular issue).
This might be useful to someone else (for this zooming-feature can be almost crippling to your work-flow when you don't know how to turn it off again). The solution was found randomly: today I accidentally but consciously did the hand-move which triggers the zooming, and I had the presence of mind of stopping to think of what I did and how to reverse it, and will describe it now.
The facts of the matter are the following:
- In Ubuntu Version
Ubuntu 16.04.3 LTS
release 16.04, Codename xenial, on my system, the following is true:
if I hold down the Alt key and simultaneously put exactly two fingers on the touchpad, then sliding the fingers towards the keyboard results in a zoom-in effect, while sliding the fingers away from the keyboard results in a zoom-out effect. The zoom-effect persists after taking all fingers away from the machine (this is the reason why this feature can be crippling: if you do this zooming-in move accidentally, and don't know how to reverse it, then you'll have to work with a heavily zoomed view, which slows you down significantly).
Notably, I tried this on another machine having Ubuntu 16.04.4 LTS installed; there, nothing happens if I do the above-mentioned moves. The versions differ by 1 only, and it seems that in the newer version this interesting though dangerous feature was removed.