(mac) Before some recent update (same one that lost the scroll bars... still missing btw...), if you closed the all acrobat windows/tabs, the app would still run, and to open the main window again you'd click on the icon.
Now, clicking on the icon does nothing. You have to either go into the menu with the application active and choose something that is likely to bring up a window, or open a document, or quit the app and open it again. None of those options are very good.