Busy indicator is only shown for 0.25s

[cpsievert]

The spinner only appears for 0.25s, then goes away. Problem introduced by #4062

library(shiny)

ui <- fluidPage(
  useBusyIndicators(),
  actionButton("btn", "Re-plot"),
  plotOutput("p")
)

server <- function(input, output, session) {

  output$p <- renderPlot({
    input$btn
    Sys.sleep(4)
    plot(1)
  })

}

shinyApp(ui, server)