Click it, and then try to prevent zoom-out when clicking it again.
There seems to be no mouseup event registered and even if I register one and stop propagation and prevent default, nothing helps.
Pretty sure you can’t interact with the default image viewer page at all (as in, you can’t insert a content script). You could however control the zoom level via tab properties?
So I’ve checked this out again and I can see that you really can’t interfere with the build in event handling, BUT you can modify whatever they are modifying… and the zooming of the image is controlled by changing a class name on the image element.
So what he is doing is reverting the changed class name on each click .
EDIT:
Actually I’m totally wrong , I don’t know how I missed this but the event (even though not visible in the Inspector) is registered as "click", not "mouseup" and can be blocked with e.stopImmediatePropagation().