In shiny (or R in general), when you start running a function, you generally cannot do anything else until that function completes. This means that if the user of a shiny app does something that results in a 2-minute calculation, the entire app becomes unusable and the user has to wait 2 minutes before they can interact with the app again. This has been a problem for some people, and the shiny team is currently looking into providing a solution for this. In the meantime, Joe Cheng (author of Shiny) came up with a nice workaroud. His solution will likely not work on Windows and it is not a robust fool-proof solution, but itcan get the job done.