I've tried:
Deleting WTF, AddOns and Cache folders
Turning off Hardware Cursor in game
Changing sensitivity options
Uninstalling and re-installing graphics drivers
Turning off enhance pointer precision
Running WoW.exe in different comparability modes with disable on high DPI setting on and off
Changing from Full Screen, to Windowed and back again (and vice versa)
I've been trying to play wow now again after a long time and apparently I can't use my 2 mouse buttons at the same time, and trying to do so would result in a weird bug that nothing becomes clickable neither ingame or out of game. if i go ctrl alt delete into task manager, this bug temporarily disappears. I'm using the draenor wotlk version windows 10
I have this problem only when playing the game over Steam in-home streaming. Disabling hardware cursor fixes it for me, though does increase pointer display latency.
FWIW, I am also on Windows 10 and don't experience this problem at all when I'm playing on the machine directly (no in-home streaming).
In case you use any third party mouse software, like my came with "Logitech Gaming Software". Make sure it is up to date and make sure you have acceleration off in there too. For me that was causing issues. I use Windows 10 x64. But I did fresh install, due to all issues I saw people having due to leftovers from their old operating system(driver and software related).
Also I use normal WotLK client, not one with new models. I just don't like to mess with client and cause any additional issues, if there would be any. And it looks exactly how I expect WotLK to look like.