Plasma 5
I was trying to enter some information on a website when suddenly my entire screen magnified. Everything on it became larger and parts of it are now off screen. To see these things I can move the mouse in one direction or the other in which case the desktop will shift position to allow me to view them.
I have no idea what caused it or how to fix it. Logging out or rebooting didn’t help
Try removing ~/.local/share/kscreen/* whilst not logged in on the desktop.
This should reset your visual settings.
Sounded good but didn’t work.
Create a new user, please and log in as that new user. Then see if the issue still exists for that new user. That would narrow down a system issue to some user setting.
It works properly with the new user.
Other files to try deleting while logged out of Plasma:
Various settings will be lost according to what you actually delete (other than the cache).
It’s really sad that KDE doesn’t put all its config files in one subdirectory tree instead of directly in ~/.config/. It did the right thing, using ~/.kde/ for KDE3 and ~/.kde4/ for KDE4. 
Wow. Talk about scorched Earth, but thanks. That did it. Wish I knew which part.
FWIW, the desktop zoom “feature” can be configured via System Settings > Desktop Behaviour > Desktop Effects > Zoom. This can be enabled or disabled as desired. I don’t use it, but the windows Meta and =/- keys are used to zoom in our out on my system. However it’s not persistent between desktop sessions as I have configured my DE to start with an empty session on login.
BTW, disabling the desktop effects with Shift+Alt+F12 would also inhibit desktop zoom (along with any other configured effects).
Yeah, that’s it alright.
Thanks guys. I can’t imagine having to use it like that permanently.
Been using this distro/Linux for 13 years. Imagine getting tripped up by something like that.
After seeing that comment about zoom, I checked. And it is turned on here. But I never knew about it, and I’ll probably never use it.
Well, okay, maybe I’ll try. When my wife complains that it looks like ants crawling over the screen, I can try zoom to see if that is easier for her to read.
After some further experimentation, I note that the zoom effect settings are located in ~/.config/kwinrc (refer to the ‘[Effect-Zoom]’ entry)…
[Effect-Zoom]
InitialZoom=1.728
Removing it (or at least setting the value to 1) will restore it to unity.