There has been two reports about this but I found out more about the left-alt cursor bug.

[dylanrtownes]

While I've found two forum posts already discussing the left-alt cursor bug, a bug in which the cursor that appears when you hit left alt can get stuck on your screen and you will be unable to aim your crosshair. I got the cursor bug but also the left alt upgrade skill overlay was also stuck on my screen, this was after I clicked on a second monitor while the left alt screen was up. For the remainder of the game the overlay and cursor was stuck on my screen.

 

[hillburn]

Similar problem for me. I'm on Linux i3-wm. When I try to level up a skill by pressing ALT+2 and accidentally also press the SUPER key at the same time, my window manager will try to put my mouse onto my secondary monitor, resulting in my cursor getting locked into ALT-mode when I return to the game. Cannot resolve this without quitting the game.