Shiny.addCustomMessageHandler(type, handler): the client half of
session$sendCustomMessage(type, ...) on the server. A port reaches
for this when the parent's own UI had an inline
tags$script(HTML("Shiny.addCustomMessageHandler(...)")) driving a
plain DOM effect (opening an offcanvas, say) that isn't itself a
hosted input or output.
Shiny.addCustomMessageHandler(type, handler): the client half of session$sendCustomMessage(type, ...) on the server. A port reaches for this when the parent's own UI had an inline tags$script(HTML("Shiny.addCustomMessageHandler(...)")) driving a plain DOM effect (opening an offcanvas, say) that isn't itself a hosted input or output.