I'm very new to shiny and in every example I've seen, both from the community and from Shiny's devs, I've never seen a button placed outside of the sidebarPanel (or some equivalent of the sidebarPanel depending on the UI structure).
Can anything go wrong if there is an action button in the main panel? Is there any reason to not put buttons (or control elements in general) in the main panel?
I'm just going with the classic sidebarLayout( sidebarPanel(...), mainPanel(...) ) if that matters for the answer