There is an app that need to use alt+left click, but instead the window got drag. I can find the setting for it. Like needle in haystack.
Thats indeed an interesting problem. By default every window manager catches that to move around windows.
You can change that luckily in the window manager -> window behavior settings -> window actions.
If you don't find it execute
Code:
kcmshell5 kwinoptions
to get to it directly.
Under the window actions tab you can set up what happens on Alt+LeftClick. Setting it to nothing is probably what you want.