1. [Optional] Disable compositor - with disable container window is
is black and easier to spot
2. Run applcaitonny application with XEmbed system tray icon, do not click on the icon
3. Restart KWin
a) Before: black/transparent rectangle in the top-left corner, reacts to
a) Before: black/transparent rectangle in the top-left corner, reacts to mouse click
b) After: no rectangle, mouse clicks work as expected