When using dual screens, there is a problem that when a window is opened on the other screen, it becomes extremely long.
A window that is longer than the screen is created on another screen.
Then, in order to reduce the window size, I have to manually find the window edges and click several times to reduce it one by one.
I updated Kicad to .6 and the problem happened again. Than I rebooted Kicad and it was gone. So I started experimenting and I know now when it happens.
It happens when the popup window appears on a different monitor. Here you see a popup over the schematic editor. I can open this as often as I want.