ALT key in Eagle (Linux)
EAGLE interferes with your operating system settings.
I'm using OpenSuse with KDE5 Plasma, where ALT + mouseclick resizes or moves the window. EAGLE may notice the ALT key is pressed, but a click is intercepted by Linux.

Just try to remove the ALT + mouseclick key binding. As every linux differs a little, I can't tell you how exactly to do it. For me, it's in workspace settings / window behaviour.