Switch to another tool?
I really don't know what you mean by "stop functioning". The Zoom tool will always work when selected.
Is there a specific example you have in mind? Give us the exact version of Photoshop you are using, and the version of Mac OSX or Windows.
A screenshot outlining your problem is best.
I just subscribed 2 days ago to the updated version of Photoshop CC and Lightroom...(I am not using Lightroom right now)...I have a new iMac 27 inch with Retina, OS 10.10.2 Yosemite...
Once I select a tool I do not know how to deselect it... The deselect mode is not lit up and the UNDO will only undo the latest activity but the tool is still there and will be active as soon as I use the curser...for example, I used the paint tool so whenever the cursor is placed on the photo, it will paint... even though I change photo.
Oh. I got Yosemite and CC, so let's explain how to do this.
If you are finished with the Brush Tool and you do not want to accidentally paint another picture, just click on the Hand Tool, and the Paint Tool will be deselected. You can also use the keyboard shortcut "h". When you click and drag on an over-sized image, the Hand Tool just moves it around.
When you want your Brush back, just click on it again.
As noted above when you are finished using a tool switch to the Hand Tool. To do that quickly just hit the H key ( lower case h, No Shift key) on the keyboard. I do this all the time.
There is no De/Un Select for any tool. Once selected that tool is active until another tool is selected.
That's not working on the magnetic lasso tool. I tried clicking on the hand tool and everything else and I
can't deselect it.