If clicking on the icon in the launcher should always either present the user with a window of that app or a selection of windows from that app and having the button do nothing is bad, then if the app has only one open window, and if that window has focus, clicking on the icon should open a new window.
If clicking on the icon in the launcher should always either present the user with a window of that app or a selection of windows from that app and having the button do nothing is bad, then if the app has only one open window, and if that window has focus, clicking on the icon should open a new window.