This is a follow-up to a previous question.

I now wish to add an action button “Clear” that is only shown when there are actually items selected. Here is my code, where I try to use shinyjs:

require(shiny)
require(shinyjs)

ui <- fluidPage(
    shinyjs::useShinyjs(), # needed for hidden/show
    shiny::selectizeInput(
        inputId="letters", 
        label="Restrict to Letters:",
        choices=letters, 
        selected=NULL,
        multiple = TRUE,
        options=list(
            placeholder="(all letters)"
        )
    ),
    shinyjs::hidden(actionButton("clear","Clear")) # start hidden
)

server <- function(input, output) {
    observeEvent(input$clear, {
        updateSelectizeInput(inputId = "letters",choices = "",session)
        shinyjs::hide(id="clear")
    })
    
    observeEvent(input$letters, { # is not called if the last selected item is removed
        message("length(input$letters)=", length(input$letters))
        if(length(input$subjects)>0) 
            shinyjs::show(id="clear")
        else  # no filter
            shinyjs::hide(id="clear")
    })
}

shinyApp(ui=ui, server=server)

The code has two problems:

1. shinyjs::show() is not working. It seems that the function IS called, but with no effect.

2. observeEvent(input$letters, …) is not called when I deselect the last item.

Any hint appreciated.