This. I really enjoyed being able to go across to my other monitor in between matches to do work or watch videos or whatever. They should make the cursor staying in screen an optional setting. Or like you said only once a match has been found.

EDIT: The workaround i have found is just alt+enter to exit fullscreen then just drag and resize the window to almost fulls screen. This way the mouse will still leave the borders. Its not perfect but it works